In addition to the clear explanations provided by keszler, here is a small demo that you may find useful, I hope:

use strict;
use warnings;
use Tk;

my $top = MainWindow->new;
$top->title("Radio");

my %wb; # radio buttons hash

# Create 4 radio buttons trick;
# cant't remember from where I got it :(
# -variable is set to default (see man page)
# -value is lower case of label
foreach my $label (qw/Top Left Right Bottom/) {
    my $lower = lc $label;
    $wb{$lower} = $top->Radiobutton(
        -text        => $label,
        -relief      => 'flat',
        -value       => $lower,
        -indicatoron => 0,
        -width       => 7,
        -command     => [\&cb_rbutton,$lower],
    )->pack;
}

$top->Button(
    -text => 'Flash',
    -width => 10,
    -command => [\&cb_button, \%wb],
)->pack;

Tk::MainLoop;

sub cb_rbutton {

    # Radio button selected
    my $rb = shift;
    print ucfirst($rb), " is on\n";
    return;
}

sub cb_button {

    # Button released
    my $rb = shift;

    foreach my $label ( keys %{$rb} ) {
        my $variable = $rb->{$label}->cget('-variable');

        if ( ! defined ${$variable} ) {
            # No check button selected
            print " Select a button!\n";
            last;
        }
        # When a check button is selected, all 4 buttons value will be
        # set to $label of the selected check button
        # To find out which one is selected:
        print " CB: $label -> ${$variable}\t";
        if ( ${$variable} eq $label ) {
            print "selected, flashing";
            $rb->{$label}->flash;
            print " ...\n";
        }
        else {
            print "\n";
        }
    }
    return;
}
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