Does Perl even support a hash 4 levels deep? What is the limit on that anyway?

Since multilevel hashes are really just hashes holding references to other (anonymous) hashes, I don't think there is any real limit. Try e.g. the following:

#!/usr/bin/perl

use strict;
use warnings;
use feature qw/say/;

use Data::Dumper;

my %hash = ();
my $hashref = \%hash;

for(1..255) {
    $hashref = $hashref->{$_} = {};
}

print Dumper \%hash;
[download]

This works like a charm and as expected.

Bisecting this, I found I can go up to 4028 layers of indirection before Perl segfaults at 4029. (And 4028 produces 178 MiB of output from Data::Dumper.)

Without the print call, there's no problem running it millions deep. What you have is a torture test for the Data::Dumper.

Also try with $Data::Dumper::Indent = 0; though it still segfaults.

Without the print call, there's no problem running it millions deep. What you have is a torture test for the Data::Dumper.

No, it segfaults for me at 4029 even if the Data::Dumper call is commented out. Sorry, I should've made that clearer above.

And thanks for the tip concerning $Data::Dumper::Indent; good to know!