I changed your insert position from 1 to 0, and reversed the array to give a sane menu. Also use warnings and use strict to avoid problems.

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

#set up main window display
use Tk;
my $mw = new MainWindow(-title => 'PopUp');
my @buttons = qw(A B C D E F G);

#popup menu set up
my $popupmenu = $mw->Menu(-tearoff => 0);

    foreach my $bt (reverse @buttons){
            $popupmenu->insert(0, 'command',
               -label => $bt,
               #-command => [sub {print $popupmenu->cget('text') ."\n"
+}],
               -command => [sub {print $bt ."\n"}],
          );
    }

$mw->bind("<Button-3>" => sub {$popupmenu->Popup(-popover => "cursor",
                                    -popanchor => 'nw'),
                                -command => [sub {print "Popup\n"}]});

MainLoop();
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