use strict and use warnings

These two lines make Perl catch your typos and sloppy code.

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use strict and use warnings

use strict and use warnings

use strict forces you to declare variables with my and stops several risky constructs, so a misspelled variable name becomes a compile-time error. use warnings prints messages about suspicious things such as using an undefined value. Almost every serious Perl script begins with both lines.

Note: Warnings go to STDERR, so they do not mix with normal program output.

Example: use strict and use warnings

perl

use strict;
use warnings;

my $total = 10;
my $result = eval q{ $totl + 1 };   # typo: $totl was never declared
if ($@) {
    print "Caught a strict error:\n";
    print $@ =~ /Global symbol "\$totl" requires explicit package name/ ? "  undeclared variable \$totl\n" : "  other error\n";
}
print "Total is $total\n";

# Output:
# Caught a strict error:
#   undeclared variable $totl
# Total is 10

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Common Mistakes

Leaving out use strict and silently creating global variables from typos
Ignoring warnings instead of fixing their cause
Forgetting to declare variables with my

Chapter Summary

use strict requires declared variables
use warnings reports suspicious code
Both catch typos early
Declare variables with my

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use strict and use warnings

use strict and use warnings

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