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use Parse::RecDescent;

...

print $g->weight("Pb(CO3)2"), $/;
# prints 327

  weight   : compound         { $item[1] }
  compound : group(s)         { sum( @{$item[1]} ) }
...
           |                  { 1 }
  element  : /[A-Z][a-z]*/    { $weights{ $item[1] } }
           | "(" compound ")" { $item[2] }