If you use the '-textvariable' option (instead of '-text'), you can update the variable and the widget will also update. Also, instead of calling Tk::After, use the 'repeat' method on the Label widget itself. Here is one way (I've replaced your gettime() function with a simple call to localtime() in scalar context to make the date string):

#!/usr/bin/perl -w
use strict;
use Tk;

my $top = new MainWindow;
my $text = localtime();
my $id = $top->Label(-textvariable => \$text)->pack;
$id->repeat(1000,sub{$text = localtime()});

MainLoop;
__END__
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