Actually, it's important that you don't use keys in front of the individual hashes, otherwise alternate keys get used as values in the construction of the anonymous hash.

As I as showed, but with values added you get

@A{ qw[ 1 2   4 6   ] } = qw[ a b   d f   ];
@B{ qw[ 1 2 3 4 6 7 ] } = qw[ a b c d f g ];

print join '.', 
    ( exists $A{ $_ } ? 'A:' . $_ : '   ' ), 
    ( exists $B{ $_ } ? 'B:' . $_ : '   ' ) 
    for sort keys %{ { %A, %B } };

A:1.B:1
A:2.B:2
   .B:3
A:4.B:4
A:6.B:6
   .B:7
[download]

But using the extra key statements you get

print join'.', 
    ( exists $A{ $_ } ? 'A:' . $_ : '   ' ), 
    ( exists $B{ $_ } ? 'B:' . $_ : '   ' ) 
    for sort keys %{ { keys %A, keys %B } };

A:1.B:1
A:6.B:6
   .B:7
[download]