I'm really just a beginner at Tk. Sorry. Maybe you could separate out the Radio Buttons into single units, and do something like the following:

#!/usr/bin/perl
use Tk;

my $mw = MainWindow->new;
my $lastbutton = 0;

my $rb1 = $mw->Radiobutton(-text => "Button One",
                            -value => 'button1',
                            -variable => \$rb,
                            -command => [ \&showRB, \$rb ])->pack;
my $rb2 = $mw->Radiobutton(-text => "Button Two",
                            -value => 'button2',
                            -variable => \$rb,
                            -command => [ \&showRB, \$rb ])->pack;
my $rb3 = $mw->Radiobutton(-text => "Button Three",
                            -value => 'button3',
                            -variable => \$rb,
                            -command => [ \&showRB, \$rb ])->pack;
my $rb2 = $mw->Radiobutton(-text => "Button Four",
                            -value => 'button4',
                            -variable => \$rb,
                            -command => [ \&showRB, \$rb ])->pack;

MainLoop;

sub showRB
{
     print "Arg list is @_\n";
     my $state_ref = shift;
     my $state = $$state_ref;
     $mw->messageBox(-title => 'Status',
                     -type  => 'OK',
                     -message  => "Status is :$state:. Previous State 
+was $lastbutton");
     $lastbutton = $state;
}
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