Why is use strict; commented out!? As previously mentioned, this is hiding serious errors. Specifically, you are trying to store both "1" and a hash reference at $h{b}{1}. One simple fix is to use undef instead of 1.

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

my %h;
$h{'B'}{1}=undef;
for my $a ('A'..'C'){
   for my $b (0..2){
     for my $c ('a'..'c') {
        $h{$a}{$b}{$c}=undef;
     }
   }
}
print "Data Dumper-------------------\n";
print Dumper (\%h);

print "Loop----------------\n";
foreach my $top (sort keys %h) {
    print $top =~ /^\s*$/ ? "<blank>" : $top, "\n";
    next if !ref($h{$top});
    foreach my $second (sort keys %{$h{$top}}) {
        print "...", $second =~ /^\s*$/ ? "<blank>" : $second, "\n";
        next if !ref($h{$top}{$second});
        foreach my $third (sort keys %{$h{$top}{$second}}) {
            print "......", $third =~ /^\s*$/ ? "<blank>" : $third, "\
+n";
            next if !ref($h{$top}{$second}{$third});
        }
    }
}

print "Print_tree------------------\n";
print_tree(\%h);

sub print_tree {
    my ($tree, $depth) = @_;
    $depth ||= 0;
    if ($depth > 5) {
        warn "max depth reached\n";
        return;
    }

    my $indent = '...' x $depth;
    for (sort keys %$tree) {
        print($indent, /^\s*\z/ ? "<blank>" : $_, "\n");
        print_tree($tree->{$_}, $depth+1)
            if ref($tree->{$_});
    }
}
[download]