If you want comprehensive symbolic manipulation, I'm not sure where to point you to..

But for the case described above (and feel free to use this as a starting point to write your own comprehensive symbolic manipulation module), you can do:

$string =~ s/ //gs; # remove spaces
$string =~ s/\((\w+)\*(\d+)\)\+\(\1\*(\d+))/($1\*(?{$2+$3})\)/gsx;
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Which should do only exactly the substitution you describe above. (Needs testing)