DB<116> $index{"A","a"}=1
 => 1

  DB<117> $index{"A","b"}=2
 => 2

  DB<118> $index{"B","a"}=3
 => 3

  DB<119> $index{"B","b"}=4
 => 4

  DB<120> grep { my ($k1,$k2) = split /$;/,$_; $k1 eq "A" } keys %index 
 => ("A\34a", "A\34b")

  DB<121> %set1=(A=>1)
 => ("A", 1)

  DB<122> %set2=(a=>1)
 => ("a", 2)

  DB<124> grep { my ($k1,$k2) = split /$;/,$_; $set1{$k1} and $set2{$k2} } keys %index  
 => "A\34a"