/usr/lib/urxvt/perl/digital-clock 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 | #! perl
=head1 NAME
digital-clock - display a digital clock overlay
=head1 DESCRIPTION
Displays a digital clock using the built-in overlay.
=cut
sub on_start {
   my ($self) = @_;
   $self->{overlay} = $self->overlay (-1, 0, 8, 1, urxvt::OVERLAY_RSTYLE, 0);
   $self->{timer} = urxvt::timer
                    ->new
                    ->start (1 + int urxvt::NOW) # make sure we update "on" the second
                    ->interval (1)
                    ->cb (sub {
                       $self->{overlay}->set (0, 0,
                          sprintf "%2d:%02d:%02d", (localtime urxvt::NOW)[2,1,0]);
                    });
   ()
}
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