What you've done is used a symbolic reference to create a second hash %k1.

use Data::Dumper;

   my $hash = {} ;

   $hash->{key1} = 'k1' ;
   $hash->{key1}{key2} = 'k2' ;

   print ">> $hash->{key1}\n" ;
   print ">> $hash->{key1}{key2}\n" ;

print Dumper $hash;
print Dumper \%k1;
^Z
>> k1
>> k2
$VAR1 = {
          'key1' => 'k1'
        };
$VAR1 = {
          'key2' => 'k2'
        };
[download]

As you don't have strict turned on, the value of $hash{key1} was used as the hash name in the second assignment and it was autovivified for you. And again when interpolated in the print statement.

You told the compiler you knew what you were doing (by not useing strict), so it went right ahead and did what you asked:) He he.