You have a really bad placement of your my $i = 0 line. But the reason your code dosn't work is you need to pack in the sub.

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

my @tab;

my $top = MainWindow->new();
my $nb  = $top->DynaTabFrame->pack;

my $i = 0;

for(1..6){ &add_tab()};

MainLoop();

sub add_tab {
    $i++;
    print "adding  tab $i\n";
    $tab[$i] = $nb->add(
        -caption  => 'Tab label' . $i,
        -tabcolor => 'black',
    )->pack;

    $tab[$i]->Label( -text => 'dsdss' )->pack;
}
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