You can put any valid lvalues on the left side of a list assignment. Inexistent things (array and hash elements -- including slices, deep (referenced) objects) will come to life automagically. The automatic creation of referenced objects is called autovivification.

See perldsc, perlref, perldata etc.

As Limbic~Region has pointed out, if you get less values on the right than of the left side in a hash slice assignment, the hash elements that do not have a correspondent on the right side will be created with an undef value (they will exist as far as exists / keys etc. are concerned). Try this example:

@h{'a', 'b'} = qw/ one /;

print "Exists!\n" if exists $h{b};
print "Undef!\n" unless defined $h{b};
[download]