PATCH: [perl #117101] toke.c: Make \$$1 work again.

Commit 3283393 replaced the use of isWORDCHAR (\p{Word}) in scan_ident
with isIDFIRST (\p{XIDS}). Generally this was not troublesome, since
there are other places that deal with variables matching \d, but
one use of isWORDCHAR had given digit variables an unintended special
case:
$$1 meant ${$1}, whereas $$@ or $$* were syntax errors.

This commit restores the special case for \p{POSIX_Digit} variables.