As mentioned elsewhere, you can use recursion:

sub foo
{
    my ($hash, @path) = @_;
    while (my ($k, $v) = each %$hash) {
        if (ref $v eq 'HASH') {
            foo($v, @path, $k);
        } else {
            # whatever
        }
    }
}
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You can also use "eval" to build up the loops (hand-waving a bit):

$depth = compute_hash_depth;
$code = '...';
for (reverse 1..$depth) {
    my $prev = $_-1;
    $code = "while (my (\$k$_, \$v$_) = each \$v$prev) { $code }";
}
eval $code;
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In general, when you want nested loops and don't know how many beforehand, recursion is easiest.