As others have said, it's autovivification.

As a quick way to explain why the 'c' key gets created, but not 'd' though: $a->{b}{c}{d} is looking for a 'd' key within $a->{b}{c}, so you are asserting that $a->{b}{c} must be a hashref for your program to succeed at all. So Perl makes it a hashref for you. But you're not asserting anything in particular about the nature of $a->{b}{c}{d}, so Perl doesn't make it anything.