Someone

# @substrings is currently up to over 10,000 entries
# string is given anything up to 20 times a second, and needs to be checked as it comes through as quickly as possible

@substrings = ('apple','orange','pear','cumquat','peach');
$string = "really long string which may or may not contain \$substring anywhere";

foreach $substring ( @substrings ) {
    if ( $substring ~= $string ) {
        print "Match found\n";
        last;
    }
}