POE::Component::DirWatch's file_callback parameter requires a plain coderef. The coderef you are supplying is a POE type handler

Your found_file sub should probably be:

sub found_file {
  my $file = shift;
  warn "Found '$file'\n";
}
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To do it a POE way would be to supply a postback or callback to POE::Component::DirWatch so it sends an event back to your session:

 use warnings;
 use strict;
 use POE;
 use POE::Component::DirWatch;
 POE::Session->create(
              inline_states => {
                  _start => \&init_watcher,
                  watch_dir => \&watch_dir,
                  found_file => \&found_file,
                               },
                    );


 sub init_watcher {
   my ($kernel, $heap, $session, $sender) = @_[KERNEL, HEAP, SESSION, 
+SENDER];
   warn "Queue 1 starting (session id " . $session->ID . ")";

  # watch a directory for this queue
  $kernel->yield('watch_dir', '/tmp/queue1'); }


 sub watch_dir {
     my ($kernel, $heap, $session, $queue) = @_[KERNEL, HEAP, SESSION,
+ ARG0];

# start watching the directory
    POE::Component::DirWatch->new(
                             alias => 'dirwatch',
                             directory => $queue,
                             file_callback => $session->postback('foun
+d_file'),
                             interval => 1,
                             );
}

sub found_file {
    my ($kernel, $heap, $session, $sender, $args) = @_[KERNEL, HEAP, S
+ESSION, SENDER, ARG1];

    my $file = shift @{ $args };

#    warn "Found '$file' for " . $session->ID . " (from " . $sender->I
+D . ")\n";
    warn "Found '$file' for $session->ID\n";

}

$poe_kernel->run();
exit(0);
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