You can create your own class derived from Tk::Label and override the configure method.

#!/usr/bin/perl

use warnings;
use strict;

use Tk;

{
    package Tk::MyLabel;
    use Tk::widgets('Label');
    @Tk::MyLabel::ISA = 'Tk::Label';

    Tk::Widget->Construct('MyLabel');

    sub configure {
        my ($cw, %args) = @_;
        if (my $bg = $args{-background}
            || $args{-bg}) {
            print "changing background to: $bg\n";
        }
        $cw->SUPER::configure(%args);
    }
}

my (@color, $color) = qw(blue green);

my $mw = tkinit;
$mw->packPropagate(0);
my $lab = $mw->MyLabel(
    -text       => "foo",
    -background => $color[0],
)->pack;
my $b = $mw->Button(
    -text => 'Do it!',
    -command => sub {
        $color = $color ? 0 : 1;
        $lab->configure(-background => $color[$color]);
    }
)->pack;
$mw->bind('<Control-q>', sub { Tk::exit(0) });

MainLoop;
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