And use  index to find number of lines.

>perl -wMstrict -le
"use Tk;
 use constant HEIGHT => 7;
 my $mw = tkinit();
 my $l = $mw->Scrolled(qw'Text -height', HEIGHT);
 $mw->Button(
   -text => 'add',
   -command => sub {
     $l->insert('end', localtime() . qq{\n});
     $l->insert('end', $l->index('end') . ' ');
     $l->insert('end', int($l->index('end')) . qq{\n});
     $l->delete('1.0', '2.0')
       while int($l->index('end')) > 2 * HEIGHT;
     $l->see('end');
     },
   )->pack;
 $l->pack;
 $l->insert('end', qq{hi\nthere\n});
 MainLoop();
"
[download]

And ditto choroba: Please use  <c> tags. Please see Markup in the Monastery.