[ ] only creates a shallow copy, so while $biu[0], $biu[1], $biu[2], etc contain references to different arrays, all those arrays have the same contents as @biu_yz.

That means $biu[0][$j], $biu[1][$j], $biu[2][$j], etc all hold the same reference as $biu_yz[$j].

Fix:

for my $i (0..12) {
   for my $j (0..26) {
      for my $k (0..12) {
         $biu[$i][$j][$k] = $u105 ;
      }
   }
}
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By the way, note how much more readable the Perl syntax for counting loops is over the C syntax.