It must be 1. Play the following snippet:

use strict;
use warnings;

use Tk;

my $mw = tkinit;

$mw->bind('<ButtonRelease>' => sub{
                my($widget) = @_;
                my $e = $widget->XEvent; # get event object
                # ($keysym_text, $keysym_decimal) = ($e->K, $e->N)
                print "Button number ",$e->N," was released\n"
            }
);

#ADD also this if unsure:
#$mw->bind('<ButtonRelease-1>' => sub{
#                    print "Button number ONE was released\n"
#                }
#);

MainLoop;
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L*