This prints the value on STDOUT - just added my $inp and () after MainLoop.

use strict;
use Tk;
my $inp;
my $input='Enter the site address..';
my $mw = new MainWindow;
my $frm_name= $mw ->Frame()->pack();
my $label =$frm_name -> Label(-text=>"Site Address") -> pack();
my $txt =$frm_name ->Entry()->pack();
my $button = $frm_name -> Button(-text => "Quit", -command => sub{ $in
+p=$txt->get();
print $inp; })->pack();
my $txt_area= $mw ->Text(-width=>10,-height=>5)->pack();
MainLoop();
[download]

Hope that helps. Update I see someone else is up early... and agree that if you want "quit" to quit then you need to add that as well.