The simplest way is to use split.

my @results = split /,/, $branch;
my $desiredAtt = $results[1];
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If you want the returned value to only be the ou value you might look into making it return a hash of hashes. Your return hash would point to another hash. The sub hash would have three keys, corpid, ou and o. Then you could call
$return{"dn"}{"ou"}
to only get the ou field.