This is due to the way that map does aliasing (it's essentially using a for() loop). What's happening is that the aref is being used in l-value context, and therefore is being auto-vivified into existence.

In your latter example, you're attempting to use the aref directly in r-value context, which is why it generates the error (because no auto-vivification happens).

You can simplify your test to use for(), and you'll get the same result as with map():

use warnings;
use strict;

my $x;

for (@{ $x }){
    print "$_\n";
}
[download]

...no output. To visually see that $x was auto-vivified as an array reference, you can use the ref() function:

my $x;
my @a = map $_, @{ $x };

print ref $x;
[download]

Output:

ARRAY
[download]

Disclaimer: taken verbatim from one of my previous StackOverflow answers.