my $rb1 = $mw->Radiobutton(-text => "Button One",
                            -value => 'button1',
                            -variable => \$rb,
                            -command => [ \&blacken, '$entry1'])->pack;
my $rb2 = $mw->Radiobutton(-text => "Button Two",
                            -value => 'button2',
                            -variable => \$rb,
                            -command => [ \&blacken, '$entry2'])->pack;

sub blacken
{
my $entry = shift;
$entry->configure(-backround => 'black');
#maybe you want
#$entry->configure(-state => 'disabled')
}