I'm not going to install Tk at the moment. But, \&callback does not call callback() (it just provides a code ref):

 $ perl -le 'sub callback { die "called( @_ )!" } print \&callback'
CODE(0x219b4c0)
 $ perl -le 'sub callback { die "called( @_ )!" } print \&callback("ar
+g")'
called( arg )! at -e line 1.
 $
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The Tk::Button documentation (see also Tk::callbacks) implies that -command => $codeRef will not immediately call the referenced sub.

There certainly could be other reasons for that usage to cause "pre-mature exit and crash". For example, -command => \&callback, time, would likely "crash".

I see plenty of evidence to suggest that it will not cause callback() to be called at button-creation time and no details suggesting that you have enough evidence to support that one part of your claim.

I'm not going to install Tk at the moment.

Ok, thanks for explaining. It's the () that causes premature exiting.

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $mw = new MainWindow;
$mw->geometry("400x200");

my $button = $mw->Button(-text    => "Sub test exit",
                 -command => \&callback(),  # exits prematurely
              # -command => \&callback,     # works ok
                          )->pack();

MainLoop;

sub callback{
  print "exiting\n";
  Tk::exit;
}
__END__
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---

I'm not really a human, but I play one on earth.
Old Perl Programmer Haiku ................... flash japh

$ perl -MO=Deparse -e " $foo = \&bar "
$foo = \&bar;
-e syntax OK

$ perl -MO=Deparse -e " $foo = \&bar() "
$foo = \(&bar());
-e syntax OK
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