OK I was able to reproduce (some of) your problems in 5.14 ... that's indeed pretty inconsistent.

As a first step we should check if newer versions still have these problems.

use strict;
use warnings;

warn "version $]";

my %hash=(a=>undef,b=>[]);
my $h=\%hash;

my @array=({a=>undef});
my $a=\@array;


warn "hash $hash{a}";
warn "hoa $hash{b}[0]";

warn "hashref $h->{a}";
warn "hoa ref $h->{b}[0]";

warn "array $array[1] ";
warn "aoh $array[0]{a} ";

warn "array ref $a->[1] ";
warn "aoh   ref $a->[0]{a} ";
[download]

version 5.014002 at /home/lanx/pm/warn_undef.pl line 4.
Use of uninitialized value $hash{"a"} in concatenation (.) or string a
+t /home/lanx/pm/warn_undef.pl line 13.
hash  at /home/lanx/pm/warn_undef.pl line 13.
Use of uninitialized value in concatenation (.) or string at /home/lan
+x/pm/warn_undef.pl line 14.
hoa  at /home/lanx/pm/warn_undef.pl line 14.
Use of uninitialized value in concatenation (.) or string at /home/lan
+x/pm/warn_undef.pl line 16.
hashref  at /home/lanx/pm/warn_undef.pl line 16.
Use of uninitialized value in concatenation (.) or string at /home/lan
+x/pm/warn_undef.pl line 17.
hoa ref  at /home/lanx/pm/warn_undef.pl line 17.
Use of uninitialized value $array[1] in concatenation (.) or string at
+ /home/lanx/pm/warn_undef.pl line 19.
array   at /home/lanx/pm/warn_undef.pl line 19.
Use of uninitialized value in concatenation (.) or string at /home/lan
+x/pm/warn_undef.pl line 20.
aoh   at /home/lanx/pm/warn_undef.pl line 20.
Use of uninitialized value in concatenation (.) or string at /home/lan
+x/pm/warn_undef.pl line 22.
array ref   at /home/lanx/pm/warn_undef.pl line 22.
Use of uninitialized value in concatenation (.) or string at /home/lan
+x/pm/warn_undef.pl line 23.
aoh   ref   at /home/lanx/pm/warn_undef.pl line 23.
[download]