It's to permit autovivification:

use feature ":5.14";
use warnings FATAL => qw(all);
use strict;
use Data::Dump qw(dump pp);
 
sub aaa($)
 {my ($a) = @_;
  $a->{a} = 1;
  say STDERR dump($a);
 }

aaa({});
aaa(undef);
aaa(0);
[download]

Produces:

{ a => 1 }
{ a => 1 }
Can't use string ("0") as a HASH ref while "strict refs" in use at C:\
+Phil\Perl\PerlMonks\undef2.pm line 8.
[download]