/usr/lib/urxvt/perl/readline 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 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 | #! perl
=head1 NAME
readline - improve readline editing (enabled by default)
=head1 DESCRIPTION
A support package that tries to make editing with readline easier. At
the moment, it reacts to clicking shift-left mouse button by trying to
move the text cursor to this position. It does so by generating as many
cursor-left or cursor-right keypresses as required (this only works
for programs that correctly support wide characters).
To avoid too many false positives, this is only done when:
=over 4
=item - the tty is in ICANON state.
=item - the text cursor is visible.
=item - the primary screen is currently being displayed.
=item - the mouse is on the same (multi-row-) line as the text cursor.
=back
The normal selection mechanism isn't disabled, so quick successive clicks
might interfere with selection creation in harmless ways.
=cut
use POSIX ();
my $termios = new POSIX::Termios;
sub on_init {
   my ($self) = @_;
   $self->{enabled} = 1;
   push @{ $self->{term}{option_popup_hook} }, sub {
      ("readline" => $self->{enabled}, sub { $self->{enabled} = shift })
   };
   ()
}
sub on_button_press {
   my ($self, $event) = @_;
   $self->current_screen || $self->hidden_cursor || !$self->{enabled}
      and return;
   my $mask = $self->ModLevel3Mask | $self->ModMetaMask
            | urxvt::ShiftMask | urxvt::ControlMask;
   ($event->{state} & $mask) == urxvt::ShiftMask
      or return;
   $termios->getattr ($self->pty_fd)
      or return;
   $termios->getlflag & &POSIX::ICANON
      and return;
   my ($row, $col) = $self->screen_cur;
   my $line = $self->line ($row);
   my $cur = $line->offset_of ($row, $col);
   my $ofs = $line->offset_of ($event->{row}, $event->{col});
   $ofs >= 0 && $ofs < $line->l
      or return;
   my $diff = $ofs - $cur;
   my $move;
   if ($diff < 0) {
      ($ofs, $cur) = ($cur, $ofs);
      $move = "\x1b[D";
   } else {
      $move = "\x1b[C";
   }
   my $skipped = substr $line->t, $cur, $ofs - $cur;
   $skipped =~ s/\x{ffff}//g;
   $self->tt_write ($move x length $skipped);
   1
}
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