[perl #130628] “so” is not a valid token name (token so { abc })

[via RT]

# New Ticket Created by  Aleks-Daniel Jakimenko-Aleksejev 
# Please include the string:  [perl #130628]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=130628 >


Code:
grammar A { token TOP { <so> }; token so { foo } }; say A.parse('foo')

Result:
Too many positionals passed; expected 1 argument but got 2
  in any !reduce at 
/tmp/whateverable/rakudo-moar/58226059b53853134de0f265b6f46a923d2004e7/share/nqp/lib/QRegex.moarvm
 line 1
  in any !cursor_pass at 
/tmp/whateverable/rakudo-moar/58226059b53853134de0f265b6f46a923d2004e7/share/nqp/lib/QRegex.moarvm
 line 1
  in regex so at /tmp/1zrz_lKZXc line 1
  in regex TOP at /tmp/1zrz_lKZXc line 1
in block <unit> at /tmp/1zrz_lKZXc line 1



Code:
grammar A { token TOP { <kuso> }; token kuso { foo } }; say A.parse('foo')

Result:
｢foo｣
 kuso => ｢foo｣