use Data::Dumper;

my %empty_hash = ();

if (exists $empty_hash{foo}) {
	print "'foo' exists\n";
}

if (exists $empty_hash{foo}{bar}) {
	print "'foo'->'bar' exists\n";
}

print Dumper \%empty_hash;

if (exists $empty_hash{foo}) {
	print "Oh look, now 'foo' exists\n";
}