in reply to Arbitrarily Nested HoH

I'd make the second argument optional, so that the initial call to your subroutine is as simple as it could be.

output_hash(\%data);

sub output_hash {
    my ($href, $item) = @_;
    die "Not a hashref" if ref($href) ne 'HASH';
    $item ||= [];

    ...
}

[download]

In Section Seekers of Perl Wisdom