Oh, you're so very close! Try the following, instead:

use strict;
use warnings;
use Data::Dumper;

sub PopulateHash {
    my $hashParm = shift;
    %$hashParm = ( 
                1 => 'a',
                2 => 'b',
                3 => 'c',
                4 => 'd');
    print Dumper(\%$hashParm);
}

my %hash = ();
PopulateHash(\%hash);
print Dumper(\%hash);
[download]

Output:

$VAR1 = {
          '4' => 'd',
          '1' => 'a',
          '3' => 'c',
          '2' => 'b'
        };
$VAR1 = {
          '4' => 'd',
          '1' => 'a',
          '3' => 'c',
          '2' => 'b'
        };
[download]

Just use the dereferenced parm that was sent to populate your hash...