tested in 5.22 with same results
version 5.022000 at pm/warn_undef0.pl line 4.
Use of uninitialized value $hash{"a"} in concatenation (.) or string a
+t pm/warn_undef0.pl line 13.
hash at pm/warn_undef0.pl line 13.
Use of uninitialized value in concatenation (.) or string at pm/warn_u
+ndef0.pl line 14.
hoa at pm/warn_undef0.pl line 14.
Use of uninitialized value in concatenation (.) or string at pm/warn_u
+ndef0.pl line 16.
hashref at pm/warn_undef0.pl line 16.
Use of uninitialized value in concatenation (.) or string at pm/warn_u
+ndef0.pl line 17.
hoa ref at pm/warn_undef0.pl line 17.
Use of uninitialized value $array[1] in concatenation (.) or string at
+ pm/warn_undef0.pl line 19.
array at pm/warn_undef0.pl line 19.
Use of uninitialized value in concatenation (.) or string at pm/warn_u
+ndef0.pl line 20.
aoh at pm/warn_undef0.pl line 20.
Use of uninitialized value in concatenation (.) or string at pm/warn_u
+ndef0.pl line 22.
array ref at pm/warn_undef0.pl line 22.
Use of uninitialized value in concatenation (.) or string at pm/warn_u
+ndef0.pl line 23.
aoh ref at pm/warn_undef0.pl line 23.