If I understand it correctly, you create them in tab order. Then follow up by packing them the way you want them to be laid out with pack, grid or whatever.

Update: Run this and hit the tab key once the Tk window comes up.

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

my $main = MainWindow->new(-title => 'Focus Test');

my $first = $main->Entry(-text => \1);
my $second = $main->Entry(-text => \2);
my $third = $main->Entry(-text => \3);

$third->pack(-side => 'left');
$second->pack(-side => 'left');
$first->pack(-side => 'left');

MainLoop;
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