Who spot the trick?

use strict;
use warnings;
my @unv;
my $pos;

while (<DATA>){push @unv,$.-1,$.,$.+1 if /XXXXX/;$pos=(tell DATA)-leng
+th $_ unless $pos}

seek DATA,$pos,0;


while (<DATA>) {print unless $.-__LINE__ ~~ @unv}
__DATA__
XXXXX
Aoooo
XXXXX is my name
Boooo
11111
22222
33333
Coooo
XXXXX is what I play
Doooo
44444


# OUTPUT

11111
22222
33333
44444
[download]

If you spot the trick please explain it to... me!
L*

UPDATE: the trick can be avoided with a cleaner solotion assigning to $.

use strict;
use warnings;

my @unv;
my $pos;

while (<DATA>){push @unv,$.-1,$.,$.+1 if /XXXXX/;$pos=(tell DATA)-leng
+th $_ unless $pos}
seek DATA,$pos,0;

$.=0;
while (<DATA>) {print unless $. ~~ @unv}
[download]

L*