As jwkrahn had pointed out, the string is not clobbered by his code. You need to supply the d flag to actually delete the characters.

knoppix@Microknoppix:~$ perl -E '
> $str = q{3 * (4 + 5) / (6 - 7)};
> say $str;
> $lb = $str =~ tr/(//;
> $rb = $str =~ tr/)//;
> say qq{left : $lb};
> say qq{right: $rb};
> say $str;
> $dig = $str =~ tr/0-9//d;
> say qq{digit: $dig};
> say $str;'
3 * (4 + 5) / (6 - 7)
left : 2
right: 2
3 * (4 + 5) / (6 - 7)
digit: 5
 * ( + ) / ( - )
knoppix@Microknoppix:~$
[download]

I hope this is helpful.