use Package1;
use Data::Dumper;

my Package1 $p = Package1->new;
$p->[0]->{attr4} = 4;
$p->[$p->[0]->{attr4}] = "set_4";
print $p->[0]->{attr4};#the above two steps sunccessfully modified the internal pseudo hash
print Dumper($p);#again proves that I successfully modified the internal pseudo hash
#print $p->{attr4};#this fails