To me, recursion means "let the programming language manage the stack", so the use of either an explicit stack or queue bothers me when I'm trying to do things recursive-like.

So here's a somewhat more fully iterator-style solution that uses the dirhandle as an iterator, and keeps a single scalar as the memory between invocations. Oh, and this isn't totally necessary, but I convert simple files to iterators that return the file once. I'm doing that because whenever I write iterators, I always use pretty much the same pattern: compute the next value if needed, consume a result from a sub-iterator, then return it. This allows a class hierarchy of iterators that override only the minimum necessary operations. Anyway, here's the code:

use IO::Dir;
sub make_reader2 {
    my ($dirname) = @_;
    my $d = new IO::Dir $dirname or die "opendir $dirname: $!";
    my $next;

    return sub {
        while (1) {
            unless (defined $next) {
                my $entry = $d->read or return;
                next if $entry eq '.';
                next if $entry eq '..';
                $entry = "$dirname/$entry";
                if (-d $entry) {
                    $next = make_reader2($entry);
                } else {
                    $next = sub { my $r = $entry; undef $entry; $r; };
                }
            }

            my $result = $next->();
            return $result if defined $result;
            undef $next;
        }
    };
}

my $reader = make_reader2($ARGV[0]);
print "$_\n" while (defined($_ = $reader->()));
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