Use the configure method to change a widget:

#!/usr/bin/perl
use warnings;
use strict;

use Tk;


sub quit_button {
    my $button = ${+shift};
    $button->configure(-text    => 'Quit',
                       -command => sub { exit });
}


sub flip_button {
    my $button = ${+shift};
    $button->configure(-text => 'Press me again',
                       -command => [ \&quit_button, \$button ]);
}


my $mw = 'MainWindow'->new;

my $button;
$button = $mw->Button(-text    => 'Press me',
                      -command => [ \&flip_button, \$button ],
                     )->pack;


MainLoop();
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