I found something on the net at http://forums.devshed.com/perl-programming-6/perl-tk-how-to-call-the-event-s-object-entry-630717.html, and also got some confirmation from pg 273 of the book Learning Perl/Tk. Instead of specifying the individual entry boxes, one should use the Tk::Entry class.

use strict;
use warnings;

use Tk;

my ($mw,$EntryBox1,$EntryBox2,$variable1,$variable2,$entry,$variable_p
+revious,$variable_current);

$mw = MainWindow->new;
$EntryBox1 = $mw->Entry(-textvariable => \$variable1,
        )
        ->place(-relx => 0.06, -rely => 0.03);

$EntryBox2 = $mw->Entry(-textvariable => \$variable2,
        )
        ->place(-relx => 0.06, -rely => 0.20);

$mw->bind("Tk::Entry","<FocusIn>",\&save_previous_value);
$mw->bind("Tk::Entry","<FocusOut>",\&compare_values);

MainLoop;

sub save_previous_value {
$entry = shift;
print "$entry\n";
# i was hoping this would print '$EntryBox1' or '$EntryBox2'
# instead it gives me 'Tk::Entry=HASH(0x1bf2f7c)'
# or 'Tk::Entry=HASH(0x1bfa9a4)'
$variable_previous = $entry->get();
      }

sub compare_values {
$entry = shift;
$variable_current = $entry->get();
if ($variable_current ne $variable_previous) {
$mw->messageBox(-message => "The value has been changed.",-type => "ok
+");
       }
       }
[download]

Now the common sub works for all the entry widgets in the main window. I would however like to know the name of the concerned entry widget. Is there some way of finding that out?