What rather surprises me is the order of errors.¹

In the first call Perl accepts passing an yet undefined hash value for the key "not_existing". E.g. you may want to populate it in foo!²

In the second call it warns about an undefined key. Keys are strings not undef which must be mapped to "".

Different things.

update

¹) ah ok, now that I can test it's obvious why! Both warnings are for the second call. The first doesn't cause warnings.

²) demonstration of a legal use case

  DB<102> sub populate { $_[0]="xxx" }

  DB<103> populate $hash{not_existent}
 => "xxx"

  DB<104> \%hash
 => { not_existent => "xxx" }

  DB<105> use warnings; populate $hash{+ undef}
Use of uninitialized value in hash element at ...
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