$i=1;
while ($i<=4);
{
  chdir ($i) or die "Konnte Verzeichnis $i nicht öffnen";
  foreach ("T1_Map","T2_Map", "PD_Map", "T2STAR_Map")
  {
    system ("fslreorient2std $_.nii.gz $_._swapped.nii.gz")
    system ("bet $_._swapped.nii.gz $_._swappedbrain.nii.gz -m");
  }
  chdir ("..");
}

continue
{
$i ++;
}