This code works OK, even if I change it to use Mechanize instead of UserAgent, but does not work inside my original program... remove_handler() removes all available handlers, including the internal, ignoring %matchspec.

  print "Hdl1: ".join(" ",$mech->handlers('response_header', $mech->re
+sponse()))."\n";
  $mech->add_handler(response_header => \&response, owner => "vitoco")
+;
  print "Hdl2: ".join(" ",$mech->handlers('response_header', $mech->re
+sponse()))."\n";
  $mech->add_header('If-Modified-Since', time2str($current)) if $curre
+nt;
  my $resp = $mech->get($url, ':content_file' => "$file.tmp");
  $mech->delete_header('If-Modified-Since') if $current;
  $mech->remove_handler('response_header', owner => "vitoco");
  print "Hdl3: ".join(" ",$mech->handlers('response_header', $mech->re
+sponse()))."\n";
  print "Hdl4: ".join(" ",$mech->handlers('response_header', $resp))."
+\n";
[download]

Hdl1: HASH(0x1f87dbc)
Hdl2: HASH(0x1f87dbc) HASH(0x1f9d174)
Hdl3:
Hdl4:
[download]

I changed de example to use :content_file, and stil works, so it is not that. Note that Hdl3 and Hdl4 uses different response objects (or the same, obtained from different ways).