Update: My apologies! Forget what I have said, it was my observation was off, not yours. But it might be a better idea to demo your issue by using Data::Dumper.

use Data::Dumper;

use strict;
use warnings;
my  %foo = ( BAZ => 1 );
sub see_args { };

print Dumper(\%foo);
see_args( @foo{qw(BAR BAZ)} );
print Dumper(\%foo);
[download]

It prints:

$VAR1 = {
          'BAZ' => 1
        };
$VAR1 = {
          'BAZ' => 1,
          'BAR' => undef
        };
[download]

Yes, autovivification is in working.

Your observation was off. In your original post, you were confused by the fact that the $ENV{TMPDIR} "appears" to have a value as you saw it through that sprintf. No, you didn't see the value of $ENV{TMPDIR}. Your sprintf printed a constant "<absent>", not the value of $ENV{TMPDIR}.</strick>