I'm preparing to present an intro to Perl for some coworkers so I'm glad I found this problem before presenting. The regex in line 10 matches when it shouldn't. I had to stare at it for a while before figuring it out. I'm posting here to give the newbies or anyone else a chance to debug a silly problem.

use warnings;
use strict;

$_ ='';

# find empty lines.
print "  - empty line\n" if /^$/;

# find lines that contain one or more whitespace characters.
print "  - contains whitespace 1\n" if m/^\s+$/ =~ $_;   ## line 10
print "  - contains whitespace 2\n" if m/^\s+$/;

[download]

Running deparse on the script makes it obvious:

<Reveal this spoiler or all in this thread>

In reply to regex matches when it seems that it shouldn't by Lotus1

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