perl -MPOSIX=setuid -e '
  sub show { printf qq[%s: real: %d, effective: %d\n] , $_[0] , $< , $> }
  show( q[current] );
  $id = 65535; #  Change to the desired value.
  setuid( $id ) or die qq[cannot change to $id: $!];
  show( q[post-change] )
  '