As matija rightly points out, you'd want to initialize your TreeNodes hash value to an actual arrayref (which would be easy to create with the anonymous arrayref constructor []).

Update: here's an example if you want to go the explicit route:

my %hash = (
    TreeNodes => []
);
push @{ $hash{TreeNodes} }, 1, 2;

use Data::Dumper;
print Dumper \%hash;
[download]

You do not even have to initialize the key, however. You can simply push into a previously non-existent key, and it will automatically be created and populated with an array reference. This is known as autovivification, and is made for situations like this.

Quick example:

my %hash;
push @{ $hash{TreeNodes} }, 1, 2;

use Data::Dumper;
print Dumper \%hash;
[download]

Update: please see perlreftut and perlref for more information about references. (Re-reading your original question leads me to believe you might still be struggling with this topic. Hopefully I haven't assumed too much, but it's a common problem, so the docs might help.)