I wrote this code assuming the regexp would still be faster, and I wanted to prove it. I was shocked by what I found. I'm borrowing some of PodMaster's benchmark code. If there's something wrong with my testing let me know, but it sure looks like the index solution blows the regexp out of the water.

#!/usr/bin/perl
use strict;
use warnings;
 
use Benchmark 'cmpthese';
use List::Util 'first';
 
my @words = qw[ new old number start simple cross heavy die exit ];
my $string = join ' 0\4/f ', map( { rand $_ } 1 .. 60), map { $words[ 
+rand @words ] } 1 .. 20;
 
cmpthese (-3, {
        regexp => sub {
            return $string =~ /(new|old|number|start|simple|cross|heav
+y|die|exit)/i;
        },
        use_index => sub {
            my $lcstring = lc $string;
            return first {index($lcstring, $_) > -1} @words;
        }
});

__END__
             Rate    regexp use_index
regexp     1084/s        --      -96%
use_index 29469/s     2619%        --
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