As LanX observes, there are really two separate issues here:

(1) $bar{not_existing} vs. $bar($baz}

$bar{undef()} generates the same Use of uninitialized value in hash element error. The difference is that $bar{not_existing} only happens (in this case) to access a non-existent element, but $bar($baz} (with $baz undefined) necessarily does so. So the latter case is guaranteed to fail, making it safe to issue a warning; but the former case might arise within otherwise-valid programming logic, so a warning is not issued. Well, that’s my guess, anyway.

(2) use of uninitialized value in scalar assignment within sub foo

I don’t know what makes the difference here, but I note that under some circumstances the warning goes away:

#! perl
use strict;
use warnings;
use Data::Dump;

sub foo
{
    dd \@_;
    my $val = $_[0];
}

my %bar;
my $baz;

foo( $bar{$baz} );
[download]

Output:

18:39 >perl 814_SoPW.pl
Use of uninitialized value $baz in hash element at 814_SoPW.pl line 28
+.
[undef]

18:39 >
[download]

Maybe a product of the way aliasing works?

Hope that helps (a little),