A few things to help you out:

• Your code doesn't compile, because there is a semi-colon missing at the end of @array = $form =~ /[A-Z][a-z]?|\d+/ig
• You assign the result of getweight(), which is in list context, to a scalar. As a result, $weight just equals 6 (the number of elements in the array). If I were you, I'd skip the subroutine and array and make weight a hash directly from the get-go:

#!/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
}
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