#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';
use Data::Printer;

print "Enter a chemical formula: ";
chomp( my $form = <> );

my %weight = $form =~ /[A-Z][a-z]?|\d+/ig;
p %weight;

__END__

Enter a chemical formula: C12H22O11
{
    C   12,
    H   22,
    O   11
}