Both the hash constructor ({}) and the array constructor ([]) create a fresh variable and a fresh reference to it.

"TEST", 42 and undef do not create anything; they simply place a previously-constructed scalar on the stack. Changing these scalars would be bad, so they are protected from being changed.

For example, take the following code:

for (1..2) {
   for ((), 4..6) {
      say ++$_;
   }

   say "";
}
[download]

Before 5.20, it output the following:

5
6
7

6
7
8
[download]

That is something that isn't read-only but should be. If "TEST", 42 or undef weren't read-only, a similar problem would occur.^[1]

There's no such problem with the newly created references from {} and []. {} and [] are akin to "<$x>" (also not read-only), not "TEST".

1. It would be particularly bad if the scalar returned by undef wasn't read-only. That particular scalar (&PL_sv_undef) is used all over the place. For example, if ++undef didn't fail, it would cause read to start returning true on failure.