The string is based on the referenced variable.

• If the variable is a hash, the string is HASH.
• If the variable is an array, the string is ARRAY.
• If the variable contains an array, the string is REF.
• ...
• Otherwise, the string is SCALAR.

So,

1. You are printing a reference to $a, which contains "foo": SCALAR
2. You are printing a reference to $a, which contains \$a: REF
3. You are printing a reference to $b, which contains "foo": SCALAR
4. You are printing a reference to $a, which contains \$a: REF