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/);
    }
}