toke.c:S_is_existing_identifier: Handle $_ @_

This calls scan_ident(), but that turns out to not handle these.  Yet
they are common constructs that we should consider.  Add special
handling for them.

Author: khwilliamson

toke.c

@@ -4476,6 +4476,16 @@ S_is_existing_identifier(pTHX_ char *s, char *e, bool is_utf8)
 
     ptrdiff_t size = e - s;
 
+    /* Special case $_ and @_ because scan_ident() doesn't recognize them */
+    if (   LIKELY(sigil == '$' || sigil == '@')
+        && size >= 2 && s[1] == '_' && s[0] == sigil
+           /* Make sure is the entire identifier, and not just a prefix to a
+            * longer one */
+        && ! isIDCONT_lazy_if_safe( (U8 *) s + 2, (U8 *) e, UTF))
+    {
+        return 2;
+    }
+
     /* khw: scan_ident appears to need a 2-byte minimum; easiest to just add
      * that than calculate a minimum, Use 3 because we offset the output one
      * byte to the right */