Other type can also be ref or deref

$coderef   = \&handler;
$globref   = \*foo

copy: 
$aref2 = [@{$aref1}]; deref + put in a array ref
$href2 = {%{$href1}}; deref + put in a array ref

Make thing exists automatically...if you access them and can produce errors

Can use: use warnings NONFATAL => 'all', FATAL => 'uninitialized' or simply use warnings FATAL => 'uninitialized' 
or use Hash::Util qw( lock_keys unlock_keys );

my $some_ref = { akey => { deeper => 1 } };
lock_keys %$some_ref;