hmm.. I smell a chance to test my understanding. Creating and storing qr// expressions takes extra work, but beats this simpler form:

my %hash_one = {
    'string_one'    => 0,
    'string_two'    => 0,
    'string_three'  => 0,
};

@ARGV = qw(file.txt);
close ARGV;

while (<>) {
    for $key (keys %hash_one) {
        $hash_one{ $key }++ if (/$key/);
    }
}
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because qr// lets perl precompile the regexp. That would pay off in cases like this, where we'll be looping through the same set of regexps over and over again, yes?