sub foo {
  return 'hello' unless $_[0];
}
print "First: " . foo(1) . "\n";
print "Second: " . foo(0) . "\n";
[download]

I have to admit I did not expect this, I expected to get nothing in the first case, not the value of the condition.

This comes from a test script for XML::Rules in which, in a filter, I had the following code:

my $parser_remove = XML::Rules->new(
    rules => [
        _default => 'raw',
        line => sub {
            my ($tag, $attrs, $context, $parents) = @_;
            if ($attrs->{_content} =~ /\s+NOT\s+/) {
                $parents->[-2]{_remove} = 1; # skip the <lines> and se
+t the attribute directly in <address>
            }
            return [$tag => $attrs];
        },
        address => sub {
            return $_[0] => $_[1] unless ($_[1]->{_remove});
        }
    ],
    style => 'filter',
);
[download]

It should remove all <address> tags that contain a <line> that contains \s+NOT\s+. Instead of just removing the said tags the code replaces them by "1", that is the value of the $_[1]->{_remove}. It did not take long to find the problem in this case, but still I think it's worth noting, in case someone makes the same mistake in a more complex code.