If I understand your question, you are using the simplest, default configuration for your two listboxes, which means that when you select an item in one box, the current selection in the other box goes away; and you want to know which listbox has a currently selected item. Just check which box has a non-empty "curselection".

Here is a really simple-minded extension of the code you posted, which should demonstrate what you're looking for:

#!/usr/bin/perl

use strict;
use Tk;

my @list1 = qw/one two three four five six seven eight nine/;
my @list2 = qw/green blue red yellow orange black brown white purple/;

my $list_used;
my $mw = MainWindow->new;
my $lb1 = $mw->Scrolled('Listbox')->pack;
my $lb2 = $mw->Scrolled('Listbox')->pack;
my $b = $mw->Button(-text => "do it", -command => \&do_it)->pack;

$lb1->insert( 'end', @list1 );
$lb2->insert( 'end', @list2 );

MainLoop;

sub do_it
{
    if ( $lb1->curselection ) {
        print " got item from lb1: ",$lb1->get($lb1->curselection),$/;
    }
    elsif ( $lb2->curselection ) {
        print "  got item from lb2: ",$lb2->get($lb2->curselection),$/
+;
    }
    else {
        print "Nothing was selected\n";
    }
}
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