my @A = (1,2,3,4,5,4,3,2,1,2);
my @B = (2,4,6,8,10,8,6,4,2);
my (%First, %Second);

push @{ $First{  $A[$_] } }, $_ for (0..$#A);
push @{ $Second{ $B[$_] } }, $_ for (0..$#B);

my %Pairs;
for my $Common (grep {exists $Second{$_}} keys %First){
  for my $Idx (@{ $First{$Common} }){
    push @{ $Pairs{$Common} }, map([$Idx, $_], @{ $Second{$Common} });
  }
}

# Print out what we found
for (sort keys %Pairs){
  print "$_:\n", map("  $_\n", map(join('-', @$_), @{ $Pairs{$_} })), "\n";
}

##</code><code>##

push @{ $First{  $A[$_] } }, $_ for (0..$#A);
push @{ $Second{ $B[$_] } }, $_ for (0..$#B);

##</code><code>##

  for my $Common (grep {exists $Second{$_}} keys %First){

##</code><code>##

  for my $Idx (@{ $First{$Common} }){
    push @{ $Pairs{$Common} }, map([$Idx, $_], @{ $Second{$Common} });
  }

##</code><code>##

  $Pairs{Each Common Value} = [ [A1, B1], [A1, B2], [A2, B1], [A2, B2] ]

##</code><code>##

  $Pairs{4} = [[3,1], [3,7], [5,1], [5,7]]

##</code><code>##

2:
  1-0
  1-8
  7-0
  7-8
  9-0
  9-8
  
4:
  3-1
  3-7
  5-1
  5-7