Now that I understand your concern, I remembered that we discussed it before...

see Unexpected warning

Actually I don't see the practical problem to get two instead of just one warning.

And if you planed to switch off the first warning, I can't find a way the second wouldn't be automatically suppressed too.

  DB<143> use warnings; no warnings 'uninitialized';  $h{+undef}
 => undef

  DB<144> sub tst { my $x=$_[0] }

  DB<145> use warnings; no warnings 'uninitialized'; tst $h{+undef}
 => undef

  DB<146> use warnings;  tst $h{+undef}
Use of uninitialized value in hash element at (eval 108)[multi_perl5db
+.pl:644] line 2.
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So what's the problem?