for (INIT; COND; INCR) {
    ...
}

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

use Tk;

my $mw = MainWindow::->new();

my @colours = qw{magenta red blue green darkred};

my $swatch = $mw->Label(-bg => 'black')->pack(-fill => 'x');

for my $i (0 .. 4) {
    $mw->Button(
        -bg => $colours[$i],
        -command => sub {
            colour_swatch(\$swatch, \@colours, \$i);
        },
    )->pack(-fill => 'x');
}

sub colour_swatch {
    my ($swatch_ref, $colours_ref, $i_ref) = @_;

    my $bg = ${$colours_ref}[$$i_ref];

    $$swatch_ref->configure(-bg => $bg);

    return;
}

MainLoop;