#!/usr/bin/perl
use strict;
use warnings;

# whatever you need to fill %weights
...

sub mol_form {
    my $formula = shift;
    my $mass;
    for ($formula =~ /[A-Za-z]\d*/g) {
        /(?<element>[A-Za-z])(?<amount>\d+)?/;
        $mass += ($+{amount} // 1) * $weights{$+{element}};
    }
    return $mass;
}

say mol_form "H2O";