use strict;
use warnings;

use 5.010;

my %molecules = (
   a => 3,
   b => 2,
   c => 3,
   );

my $globStr =
   join
      q{},
      map { qq{{@{ [ join q{,}, 1 .. $molecules{ $_ } ] }}} }
      sort keys %molecules;

say for glob $globStr;