To add to the next comments, if you want to see what string is longer, use length  length $st1 so you can check the longest one, as i understand you need that

#so you get something like this:
my $index = 0; 
if ( lenght $str1 < length $str2 ) {
   $index= index $str2,str1;
} else {
   $index= index $str1,str2;
}
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