Show us a bit more of your own code.

Ok, I first:

package PUA;
use LWP::Parallel::UserAgent;
@PUA::ISA = qw(LWP::Parallel::UserAgent);

sub on_return {
    # print '@_ = (' . join(', ',map{"'$_'"} @_) .")\n";
    my ($self,$request, $response, $entry) = @_;
    $self->discard_entry($entry);
} 
1;
[download]

#!/usr/bin/perl
use strict;
use Data::Dumper;
$Data::Dumper::Indent = 1;
use HTTP::Request;
use PUA;
my $ua = PUA->new();
for(<*.pl>) {
    my $request = HTTP::Request->new('GET',"file://$ENV{HOME}/perlmonk
+s/$_");
    if( my $res = $ua->register($request)) {
        print STDERR $res->error_as_HTML;
    } 
}
print Dumper($ua);
[download]

Output:

$VAR1 = bless( {
  'ordpend_connections' => [],
  'entries_by_sockets' => {},
  'max_hosts' => 7,
  'seen_request' => {},
  'handle_duplicates' => 0,
  'requests_redirectable' => [
    'GET',
    'HEAD'
  ],
  'from' => undef,
  'timeout' => 180,
  'handle_response' => 1,
  'parse_head' => 1,
  'remember_failures' => 0,
  'entries_by_requests' => {},
  'max_req' => 5,
  'current_connections' => {},
  'max_redirect' => 7,
  'nonblock' => 0,
  'previous_requests' => {},
  'select_out' => bless( [
    undef,
    0
  ], 'IO::Select' ),
  'pending_connections' => {},
  'failed_connections' => {},
  'protocols_forbidden' => undef,
  'no_proxy' => [],
  'protocols_allowed' => undef,
  'use_eval' => 1,
  'agent' => 'libwww-perl/5.805',
  'handle_in_order' => 0,
  'def_headers' => undef,
  'proxy' => {},
  'select_in' => bless( [
    undef,
    0
  ], 'IO::Select' ),
  'max_size' => undef
}, 'PUA' );
[download]

Works like expected. No Request/Response object in $ua. If I comment out the $self->discard_entry($entry) line in PUA.pm, the $ua object is stuffed with the entries in the anonymous array keyed as ordpend_connections in the object's hash.

Your turn. I suspect usage errors.

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}