Thats easy enough to accomplish, since after a selection, a Delete press will delete the selection.

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

my $mw = new MainWindow; 

my $ent = $mw -> Entry(
          -bg => 'white',
          -selectbackground => 'hotpink',
        ) -> pack();

$ent->focus;

$ent->bind('<Control-a>', \&select_it  );

MainLoop;

sub select_it{
 $ent->selectionRange(0,'end');
}
[download]