I used the following script with no problems.

#!/usr/bin/perl
$x='y'; # comment this line out to test undef case.

%p = (
      'a' => 'B',
      'z' => defined($x) ? $x : 'n',
      'c' => 'D');

print $p{z}
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(BTW:When you call warn it will generate a warning with the input you give it, so I'm not surprised by the warning you got. If you intended it as a test clause then you should say warn 'err' if( !defined($x) ); or something like that.)
Oh, and yes... if you include an undef in an array it will get dropped, which is why your hash was acting funny.