Unlike pretty much any other function that returns 'false' in Perl, meaning 0 or a negative number of undef, the index() function returns the '-1' string rather than the actual number -1. Go ahead, try this out. I bet it doesn't do what you would expect: 
my $string = 'string here';
my $index = index $string, 'bah';
print $index if $index;

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Believe it or not, you actually have to force a numeric context to get it to work:
print $index if $index >= 0;
Beware of this one when using index as a faster alternative to regular expressions when simply checking for keywords in a string. The following is particularly subtle and annoying: 
if (my $index = index $string, $key) {
    # yadda
}

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That block of 'if' code will always be executed. And here I thought I'd run into my last internal perl bug, reminds me of the 90s again.

In reply to Perl index() Function Returns String, Not Numeric by ironmo

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