package my::model;

use strict;
use warnings;

sub use_subclass_package_var {
    my ($self, @cols) = @_;
    my $normal = join '::', ref $self, 'normal';

    foreach my $col (@cols) {
        #
        # I need help getting the next line to work properly
        #

        next unless exists eval { %{$normal{$col}} };

        # the rest of the code
    }
}