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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