Well, it's warning cause there's an undef on the third split :) 
  ($x, $ENV{Z1Z1}) = ( '3', '2,1' );  # first time
  ($x, $ENV{Z1Z1}) = ( '2', '1' );  # second time
  ($x, $ENV{Z1Z1}) = ( '1', undef );  # third time

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To avoid it, you can explicitly get and check the split() results: 
  my @arr = split /,/, $ENV{'Z' . uc $foo}, 2;
  $x = shift @arr;
  $ENV{'Z' . uc $foo} = scalar @arr ? $arr[0] : '';   # set to empty s
+tring if nothing available

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In reply to Re: Uninitialised value in list assignment by davidrw
in thread Uninitialised value in list assignment by Tanktalus

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