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This comprehensive guide covers: - 22 chapters of practical Perl programming - Focus on system administration and automation - Modern Perl best practices and techniques - Real-world examples and production-ready code - 3 appendices with one-liners, gotchas, and resources The book targets experienced sysadmins, DevOps engineers, and automation specialists, demonstrating Perl's continued relevance in 2025 for text processing, system administration, and rapid development. 🤖 Generated with Claude Code Co-Authored-By: Claude <noreply@anthropic.com>
623 lines
14 KiB
Markdown
623 lines
14 KiB
Markdown
# Appendix B: Common Gotchas and Solutions
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*"Experience is simply the name we give our mistakes, and Perl has taught us many valuable experiences."*
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Every Perl programmer, from novice to expert, has fallen into these traps. This appendix catalogs the most common pitfalls and their solutions, saving you hours of debugging frustration.
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## Context Confusion
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### The Problem
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Perl's scalar vs. list context is powerful but can be confusing:
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```perl
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# GOTCHA: Unexpected scalar context
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my @array = (1, 2, 3, 4, 5);
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my $count = @array; # $count is 5 (array in scalar context)
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# But this might surprise you:
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my @files = glob("*.txt");
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if (@files) { # This checks if array is non-empty
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print "Found files\n";
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}
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if (glob("*.txt")) { # GOTCHA: Only checks first file!
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print "This might not do what you expect\n";
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}
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# SOLUTION: Force list context
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if (() = glob("*.txt")) {
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print "Found files\n";
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}
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```
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### List vs. Array
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```perl
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# GOTCHA: List assignment in scalar context
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my $x = (1, 2, 3); # $x is 3, not an array!
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# SOLUTION: Use array reference
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my $x = [1, 2, 3]; # $x is an arrayref
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# GOTCHA: Returning lists
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sub get_data {
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return (1, 2, 3); # Returns list
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}
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my $data = get_data(); # $data is 3 (last element)
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my @data = get_data(); # @data is (1, 2, 3)
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# SOLUTION: Return reference for consistency
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sub get_data_ref {
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return [1, 2, 3];
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}
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my $data = get_data_ref(); # Always returns arrayref
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```
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## Reference Gotchas
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### Accidental References
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```perl
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# GOTCHA: Creating a reference when you don't mean to
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my @array = (1, 2, 3);
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my $ref = @array; # $ref is 3, not a reference!
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# SOLUTION: Use backslash
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my $ref = \@array; # Now it's a reference
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# GOTCHA: Symbolic references
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my $var_name = "foo";
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$$var_name = 42; # Creates $foo = 42 (dangerous!)
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# SOLUTION: Use strict
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use strict; # This will catch symbolic references
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# GOTCHA: Circular references
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my $node = { value => 1 };
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$node->{next} = $node; # Memory leak!
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# SOLUTION: Weaken references
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use Scalar::Util 'weaken';
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my $node = { value => 1 };
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$node->{next} = $node;
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weaken($node->{next});
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```
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### Reference vs. Copy
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```perl
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# GOTCHA: Modifying shared references
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my @original = (1, 2, 3);
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my $ref1 = \@original;
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my $ref2 = $ref1;
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push @$ref2, 4; # Modifies @original!
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# SOLUTION: Deep copy
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use Storable qw(dclone);
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my $ref2 = dclone($ref1);
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# GOTCHA: Shallow copy of nested structures
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my $original = { a => [1, 2, 3] };
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my $copy = { %$original };
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push @{$copy->{a}}, 4; # Modifies $original->{a}!
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# SOLUTION: Deep copy for nested structures
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my $copy = dclone($original);
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```
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## String and Number Confusion
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### Numeric vs. String Comparison
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```perl
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# GOTCHA: Wrong comparison operator
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my $x = "10";
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my $y = "9";
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if ($x > $y) {
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print "10 > 9 (numeric)\n"; # Correct
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}
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if ($x gt $y) {
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print "10 gt 9 (string)\n"; # Wrong! "10" lt "9" as strings
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}
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# GOTCHA: Sorting numbers as strings
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my @numbers = (1, 2, 10, 20, 3);
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my @wrong = sort @numbers; # (1, 10, 2, 20, 3)
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my @right = sort { $a <=> $b } @numbers; # (1, 2, 3, 10, 20)
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# GOTCHA: String increment
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my $ver = "1.9";
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$ver++; # Becomes 2, not "1.10" or "2.0"!
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# SOLUTION: Version objects
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use version;
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my $ver = version->new("1.9");
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$ver++; # Properly increments version
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```
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### Unexpected String Conversion
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```perl
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# GOTCHA: Leading zeros
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my $num = 0755; # Octal! Value is 493
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my $str = "0755";
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my $val = $str + 0; # 755, not 493
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# SOLUTION: Explicit conversion
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my $octal = oct("0755"); # 493
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my $decimal = "755" + 0; # 755
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# GOTCHA: Floating point comparison
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my $x = 0.1 + 0.2;
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if ($x == 0.3) { # May fail due to floating point!
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print "Equal\n";
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}
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# SOLUTION: Use tolerance
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use constant EPSILON => 1e-10;
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if (abs($x - 0.3) < EPSILON) {
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print "Equal within tolerance\n";
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}
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```
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## Regular Expression Pitfalls
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### Greedy vs. Non-Greedy
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```perl
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# GOTCHA: Greedy matching
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my $html = '<div>Content</div><div>More</div>';
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$html =~ /<div>(.+)<\/div>/;
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# $1 is "Content</div><div>More", not "Content"!
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# SOLUTION: Non-greedy quantifier
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$html =~ /<div>(.+?)<\/div>/;
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# $1 is "Content"
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# GOTCHA: Dot doesn't match newline
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my $text = "Line 1\nLine 2";
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$text =~ /Line.+2/; # Doesn't match!
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# SOLUTION: /s modifier
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$text =~ /Line.+2/s; # Now it matches
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```
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### Capture Variables
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```perl
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# GOTCHA: $1 persists after failed match
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"foo" =~ /(\w+)/; # $1 is "foo"
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"123" =~ /([a-z]+)/; # Match fails, but $1 is still "foo"!
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# SOLUTION: Check match success
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if ("123" =~ /([a-z]+)/) {
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print $1;
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} else {
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print "No match\n";
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}
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# GOTCHA: Nested captures
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"abcd" =~ /((a)(b))/;
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# $1 is "ab", $2 is "a", $3 is "b"
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# SOLUTION: Use named captures for clarity
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"abcd" =~ /(?<group>(?<first>a)(?<second>b))/;
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# $+{group}, $+{first}, $+{second}
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```
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### Special Characters
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```perl
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# GOTCHA: Unescaped metacharacters
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my $price = '$19.99';
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$price =~ /\$19.99/; # Need to escape $
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$price =~ /\$19\.99/; # And the dot!
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# SOLUTION: quotemeta or \Q...\E
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$price =~ /\Q$19.99\E/;
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# GOTCHA: Variable interpolation in regex
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my $pattern = "a+b";
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"aaaaab" =~ /$pattern/; # Looks for "a+b" literally!
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# SOLUTION: Pre-compile or use qr//
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my $pattern = qr/a+b/;
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"aaaaab" =~ /$pattern/; # Now it works
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```
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## Scope and Variable Issues
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### my vs. our vs. local
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```perl
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# GOTCHA: my in false conditional
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if (0) {
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my $x = 42;
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}
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print $x; # Error: $x not declared (good!)
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# But this is tricky:
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my $x = 10 if 0; # $x is declared but undefined!
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print $x; # Prints nothing, not an error
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# SOLUTION: Never use my with statement modifiers
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my $x;
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$x = 10 if $condition;
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# GOTCHA: local doesn't create new variable
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our $global = "global";
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{
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local $global = "local"; # Temporarily changes $global
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print $global; # "local"
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}
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print $global; # Back to "global"
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# GOTCHA: Closures and loops
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my @subs;
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for my $i (0..2) {
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push @subs, sub { print $i };
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}
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$_->() for @subs; # Prints "222", not "012"!
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# SOLUTION: Create new lexical
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for my $i (0..2) {
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my $j = $i;
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push @subs, sub { print $j };
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}
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```
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### Package Variables
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```perl
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# GOTCHA: Forgetting to declare package variables
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package Foo;
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$variable = 42; # Creates $Foo::variable
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package Bar;
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print $variable; # Undefined! Looking for $Bar::variable
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# SOLUTION: Use our or fully qualify
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package Foo;
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our $variable = 42;
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package Bar;
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print $Foo::variable;
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# GOTCHA: Package affects lexicals
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package Foo;
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my $x = 42;
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package Bar;
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print $x; # Still 42! my is lexical, not package-scoped
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```
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## File Handle Problems
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### File Handle Scope
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```perl
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# GOTCHA: Global filehandles
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open FILE, "data.txt";
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# FILE is global, can conflict!
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# SOLUTION: Lexical filehandles
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open my $fh, '<', "data.txt" or die $!;
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# GOTCHA: Not checking open success
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open my $fh, '<', "nonexistent.txt";
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while (<$fh>) { # Silently does nothing!
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print;
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}
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# SOLUTION: Always check
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open my $fh, '<', "file.txt" or die "Cannot open: $!";
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# GOTCHA: Filehandle in variable
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my $fh = "STDOUT";
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print $fh "Hello"; # Doesn't work!
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# SOLUTION: Use glob or reference
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my $fh = *STDOUT;
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print $fh "Hello";
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```
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### Buffering Issues
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```perl
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# GOTCHA: Output buffering
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print "Processing...";
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sleep 5;
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print " Done!\n"; # "Processing..." doesn't appear for 5 seconds!
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# SOLUTION: Disable buffering
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$| = 1; # Or use autoflush
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# GOTCHA: Reading and writing same file
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open my $fh, '+<', "file.txt" or die $!;
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my $line = <$fh>;
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print $fh "New line\n"; # Where does this go?
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# SOLUTION: Seek between read and write
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seek($fh, 0, 0); # Go to beginning
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```
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## Loop Pitfalls
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### Iterator Variables
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```perl
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# GOTCHA: Modifying foreach iterator
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my @array = (1, 2, 3);
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for my $elem (@array) {
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$elem *= 2; # This modifies @array!
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}
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# @array is now (2, 4, 6)
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# SOLUTION: Work with copy if needed
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for my $elem (@array) {
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my $double = $elem * 2;
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# Use $double, don't modify $elem
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}
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# GOTCHA: $_ is aliased
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my @array = (1, 2, 3);
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for (@array) {
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$_ = "x"; # Changes array elements!
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}
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# @array is now ("x", "x", "x")
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# GOTCHA: Reusing iterator variable
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for my $i (1..3) {
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for my $i (1..3) { # Shadows outer $i
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print "$i ";
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}
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print "$i\n"; # Always prints 3
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}
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```
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### Loop Control
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```perl
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# GOTCHA: next/last in do-while
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my $i = 0;
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do {
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next if $i == 5; # Doesn't work as expected!
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print "$i ";
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$i++;
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} while ($i < 10);
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# SOLUTION: Use while or for
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while ($i < 10) {
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$i++;
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next if $i == 5;
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print "$i ";
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}
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# GOTCHA: Loop label scope
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OUTER: for my $i (1..3) {
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for my $j (1..3) {
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last OUTER if $i * $j > 4; # Exits both loops
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}
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}
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```
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## Hash Surprises
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### Key Stringification
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```perl
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# GOTCHA: Numeric keys become strings
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my %hash;
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$hash{01} = "a"; # Key is "1"
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$hash{1.0} = "b"; # Also key "1"
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$hash{"1"} = "c"; # Still key "1"
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# Only one key exists!
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# GOTCHA: Reference as key
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my $ref = [1, 2, 3];
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my %hash = ($ref => "value"); # Key is "ARRAY(0x...)"
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# SOLUTION: Use Tie::RefHash or stringify manually
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use Tie::RefHash;
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tie my %hash, 'Tie::RefHash';
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$hash{$ref} = "value";
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# GOTCHA: undef as key
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my %hash = (undef, "value"); # Key is empty string ""
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$hash{""} = "other"; # Overwrites the previous value
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```
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### List Assignment
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```perl
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# GOTCHA: Odd number of elements
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my %hash = (1, 2, 3); # Warning: Odd number of elements
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# %hash is (1 => 2, 3 => undef)
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# GOTCHA: Hash slice assignment
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my %hash;
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@hash{'a', 'b'} = (1); # Only 'a' gets value!
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# %hash is (a => 1, b => undef)
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# SOLUTION: Provide all values
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@hash{'a', 'b'} = (1, 2);
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```
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## Operator Precedence
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### Common Precedence Mistakes
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```perl
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# GOTCHA: || vs or
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open my $fh, '<', 'file.txt' or die "Error: $!"; # Correct
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open my $fh, '<', 'file.txt' || die "Error: $!"; # Wrong!
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# Parses as: open my $fh, '<', ('file.txt' || die "Error: $!")
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# GOTCHA: Arrow operator precedence
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my $x = $hash->{key} || 'default'; # OK
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my $x = $hash->{key} or 'default'; # Wrong precedence!
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# GOTCHA: Ternary operator
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my $x = $cond ? $a = 1 : $b = 2; # Confusing!
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# Better:
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my $x = $cond ? ($a = 1) : ($b = 2);
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# GOTCHA: String concatenation
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print "Value: " . $x + 1; # Wrong! Tries numeric addition
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print "Value: " . ($x + 1); # Correct
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```
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## Special Variable Gotchas
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### $_ Problems
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```perl
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# GOTCHA: Unexpected $_ modification
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for (1..3) {
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do_something(); # Might change $_!
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print $_; # Not what you expect
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}
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# SOLUTION: Localize $_
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for (1..3) {
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local $_ = $_;
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do_something();
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print $_;
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}
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# GOTCHA: map/grep modify $_
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my @data = (1, 2, 3);
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map { $_ *= 2 } @data; # Modifies @data!
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# SOLUTION: Return new values
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my @doubled = map { $_ * 2 } @data;
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```
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### @_ Handling
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```perl
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# GOTCHA: @_ is aliased
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sub modify {
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$_[0] = "changed";
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}
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my $x = "original";
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modify($x); # $x is now "changed"!
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# SOLUTION: Copy parameters
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sub safe_modify {
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my ($param) = @_;
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$param = "changed"; # Doesn't affect original
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}
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# GOTCHA: shift in nested subs
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sub outer {
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my $arg = shift;
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my $sub = sub {
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my $inner = shift; # Shifts from inner @_, not outer!
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};
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}
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```
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## Module and Package Issues
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### use vs. require
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```perl
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# GOTCHA: require doesn't import
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require Some::Module;
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Some::Module::function(); # Must use full name
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# vs.
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use Some::Module;
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function(); # Imported (if module exports it)
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# GOTCHA: Runtime vs. compile time
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if ($condition) {
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use Some::Module; # Always executed at compile time!
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}
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# SOLUTION: Use require for conditional loading
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if ($condition) {
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require Some::Module;
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Some::Module->import();
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}
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# GOTCHA: Version checking
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use Some::Module 1.23; # Compile-time version check
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require Some::Module;
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Some::Module->VERSION(1.23); # Runtime version check
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```
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## Performance Gotchas
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### Unexpected Slowness
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```perl
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# GOTCHA: Repeated regex compilation
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for my $item (@items) {
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if ($item =~ /$pattern/) { # Recompiles each time if $pattern changes
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# ...
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}
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}
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# SOLUTION: Compile once
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my $re = qr/$pattern/;
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for my $item (@items) {
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if ($item =~ /$re/) {
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# ...
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}
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}
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# GOTCHA: Slurping huge files
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my $content = do { local $/; <$fh> }; # Loads entire file into memory
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# SOLUTION: Process line by line
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while (my $line = <$fh>) {
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process($line);
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}
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# GOTCHA: Unnecessary copying
|
|
sub process_array {
|
|
my @array = @_; # Copies entire array!
|
|
# ...
|
|
}
|
|
|
|
# SOLUTION: Use references
|
|
sub process_array {
|
|
my $array_ref = shift;
|
|
# Use @$array_ref
|
|
}
|
|
```
|
|
|
|
## Unicode and Encoding
|
|
|
|
### UTF-8 Issues
|
|
```perl
|
|
# GOTCHA: Forgetting to decode input
|
|
open my $fh, '<', 'utf8_file.txt';
|
|
my $line = <$fh>; # Bytes, not characters!
|
|
|
|
# SOLUTION: Specify encoding
|
|
open my $fh, '<:encoding(UTF-8)', 'utf8_file.txt';
|
|
|
|
# GOTCHA: Double encoding
|
|
use utf8; # Source code is UTF-8
|
|
my $str = "café";
|
|
print encode_utf8($str); # Double encodes if output is UTF-8!
|
|
|
|
# GOTCHA: Length of UTF-8 strings
|
|
my $str = "café";
|
|
print length($str); # Might be 4 or 5!
|
|
|
|
# SOLUTION: Decode first
|
|
use Encode;
|
|
my $decoded = decode_utf8($str);
|
|
print length($decoded); # Always 4
|
|
```
|
|
|
|
## Quick Reference: Solutions
|
|
|
|
| Problem | Solution |
|
|
|---------|----------|
|
|
| Wrong context | Force context with () or scalar() |
|
|
| Symbolic references | use strict 'refs' |
|
|
| Circular references | use Scalar::Util 'weaken' |
|
|
| String/number comparison | Use correct operators (== vs eq) |
|
|
| Greedy regex | Use non-greedy: +? *? |
|
|
| Failed match persists | Check match success |
|
|
| Global filehandles | Use lexical: open my $fh |
|
|
| Buffering delays | Set $\| = 1 |
|
|
| Iterator modification | Use copies or indices |
|
|
| Hash key stringification | Be aware of automatic conversion |
|
|
| Precedence errors | Use parentheses liberally |
|
|
| $_ clobbering | Localize with local |
|
|
| Module import issues | Understand use vs. require |
|
|
| Performance problems | Profile, don't guess |
|
|
| Encoding errors | Explicitly specify encodings |
|
|
|
|
Remember: These gotchas exist not because Perl is flawed, but because it's flexible. Understanding them makes you a better Perl programmer and helps you write more robust, maintainable code.
|
|
|
|
*"The difference between a Perl novice and a Perl expert? The expert has made all these mistakes already."* |