use Tk ':variables';
...

# It handles right-clicks on all entries of the main window
$mwindow->bind("Tk::Entry", "<Button-3>", sub {$popup->Popup(
	-popover 	=> "cursor",
	-popanchor 	=> 'nw'
	)}
);

# It makes the menu row 'Copy' and calls the sub callback
my $popup = $mwindow->Menu(
	-tearoff   		=> 0,
	-menuitems 		=> [
		['command' => "~Copy", -command => \&callback]
	]
);

# It must copy the content of the event's entry to the clipboard, but...
sub callback {
	my $f = $Tk::event->ENTRY; # what instead of ENTRY? which method? (A, B...)
	my $g = $f->get();
	$mw->clipboardAppend($g);
}