/usr/lib/urxvt/perl/clipboard-osc is in rxvt-unicode 9.20-1+b1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 | #! perl
=head1 NAME
clipboard-osc - implement the clipboard operating system command sequence
=head1 SYNOPSIS
   urxvt -pe clipboard-osc
=head1 DESCRIPTION
This extension implements the clipboard;copy Perl OSC.
=cut
sub on_osc_seq_perl {
   my ($self, $osc, $resp) = @_;
   return unless $osc =~ s/^clipboard;([^;]+)//;
   if ($1 eq "copy") {
      my $text = $self->selection ();
      $self->selection ($text, 1);
      $self->selection_grab (urxvt::CurrentTime, 1);
   }
   1
}
 |