my $asdf = $dbh->selectall_hashref($sth,['blah1','blah2','blah3']);

while ( my ($key1, $val1) = each %$asdf ) {
  while ( my ($key2, $val2) = each %$val1 ) {
    while ( my ($key3, $val3 = each %$val2 ) {

# Do something here...

    }        
  }
}

##</code><code>##

for my $key ( keys %$hashref ) {
...
}