use strict;
use warnings;


sub id2p_choroba {
  my $id = shift or return q();

  my $chunk_length = length $id > 6 ? 3 : 2;

  $id = sprintf '%0' . ($chunk_length * 3) . 'd', $id;

  my $chunk = ".{$chunk_length}";
  my $path  = join '/', $id =~ / ^ (.*) ($chunk) ($chunk) $ /xg;
  return $path;
}

sub id2p_lanx {
  my $id = shift or return q();

 
  my ($chunk,$len) = (length $id > 6 )
    ? (3,9)
      : (2,6);

  my $norm = sprintf "%0${len}d", $id;
  
  my $path = join "/",
    ( $norm =~ m/ ^ (-?\d+) (\d{$chunk}) (\d{$chunk}) $ /x );

  #  $path ||= '-00/000/001'; # this weirdness is needed if '-?' is mi
+ssing
  
  return $path;
}


use Test::More tests => 23;

is(id2p_lanx($_), id2p_choroba($_), "id=$_")
  for q(), 0, 1, 9, 10, 99,
  100, 999, 1000, 9999, 10000,
  99999, 100000, 999999, 1e6,
  1e7-1, 1e7, 1e8-1, 1e8, 1e9,
  1e10, 1e11, 1e12;
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update

hmm ... well ... surprisingly now after some modifications my code looks a lot like yours! ;-)