From @cognominal

grammar G { token TOP() { 'a' } }
class A { method TOP($/) { G.parse('a'); } }
G.parse('a', :actions(A));

=begin pod

  perl6 u
  Cannot assign to a readonly variable or a value
  in method TOP at u​:2
  in regex TOP at u​:1
  in block <unit> at u​:3

The example is pretty stupid because it would lead to a infinite recursion.
This is just a golfed version of a real example which used a grammar to drive tests
on another grammar that led to the same error.

=end pod

From @timo

The thing is that G.parse('a') puts (well, tries to put) the parse
result into $/. $/ is a part of your method's signature and by default
those are read-only. That's why you're being accused of assigning to a
readonly variable.

what you'll need to do is either call the parameter something else or
put G.parse('a') into an inner scope that defines its own $/.

I'm willing to close this as "notabug".

The RT System itself - Status changed from 'new' to 'open'

From @cognominal

On Mon Oct 05 05​:14​:25 2015, timo wrote​:

The thing is that G.parse('a') puts (well, tries to put) the parse
result into $/. $/ is a part of your method's signature and by default
those are read-only. That's why you're being accused of assigning to a
readonly variable.

what you'll need to do is either call the parameter something else or
put G.parse('a') into an inner scope that defines its own $/.

I'm willing to close this as "notabug".

Indeed nothing obliged me to use $/ as a parameter.
On the other hand, the error message does not tell what is the problematic variable.
Thx.

@cognominal - Status changed from 'open' to 'rejected'