Just a very minor point regarding your code. You don't need to tr/x/x/ when just counting characters; tr/x// will do.

knoppix@Microknoppix:~$ perl -E '
> $str = q{banana};   
> say $str;
> $cnt = $str =~ tr{a}{};
> say $cnt;
> say $str;'
banana
3
banana
knoppix@Microknoppix:~$
[download]