Maybe just a bit safer: test if it IS a hash:

sub print_tree
{
    my ($tree, $depth) = (@_, 0);
    ref $tree eq "HASH" or return;
    my $indent = "..." x $depth;
    for (sort keys %$tree) {
        print $indent, /^\s*\z/ ? "<blank>" : $_, "\n";
        print_tree ($tree->{$_}, $depth + 1);
        }      
    }

print_tree (\%categories);
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