sub button {
    my ($w, $bg, $text, $pcmd, $ival) = @_;
    (defined $ival) and print "Value of \$i is $ival\n";
    my $b = $w->Button(-bg => $bg, -text => $text);
    $b->configure(-width => 16, -command => $pcmd);
    $b->pack;
    return $b;
}

# ... and later, when calling button() ...
    push @$pbuttons, button($frame, 'skyblue', $dup, $psub, $i);

# The above correctly displays each value of $i