use Parse::RecDescent;
use List::Util 'sum';
use vars '%weights';

%weights = qw( C 12 O 16 Pb 207 );

my $g = Parse::RecDescent->new(<<'END_GRAMMAR');
  weight: compound         { $item[1] }
  compound: group(s)       { ::sum( @{$item[1]} ) }
  group: element /\d+/     { $item[1] * $item[2] }
       | element           { $item[1] }
  element: /[A-Z][a-z]*/   { $::weights{ $item[1] } }
         | "(" compound")" { $item[2] }
END_GRAMMAR

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