That's called autovivification IIRC. See:

$ pirl
Welcome to the Perl shell. Type ':help' for more information


pirl @> %a = ()
()


pirl @> print "ok" if $a{k}
undef


pirl @> \%a # no surprise
{}


pirl @> print "ok" if $a{k}[0]
undef


pirl @> \%a # bang!
{ k => [] }
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When accessing elements of a structure (hash or array ref) in a nested level, you may find out that Perl "helpfully" sprang some structures to life. If you're not ready for this, it may give a lot of headache.

Autovivification issues happen a lot here. See for example Hash Autovivification Weirdness, Re: Array mysteriously growing (autovivification explained), Autovivification of scalars in sub calls, Autovivification for dummies (using Test::More), The Bad, the Ugly, and the Good of autovivification or look for more following the super search http://www.perlmonks.org/?node_id=3989;HIT=vivification.