result

TOP
|  proto
|  |  message
|  |  |  ident
|  |  |  * MATCH "fart"
|  |  |  message-body
|  |  |  |  ident
|  |  |  |  * MATCH "ass"
|  |  |  * MATCH "\{ass }"
|  |  * MATCH "message fart \{ass }"
|  * MATCH "message fart \{ass }"
* MATCH "message fart \{ass }"
No such method 'from' for invocant of type 'Any'
  in method caps at src/gen/CORE.setting:10127
  in method gist at src/gen/CORE.setting:10158
  in method gist at src/gen/CORE.setting:893
  in method gist at src/gen/CORE.setting:10159
  in method gist at src/gen/CORE.setting:893
  in method gist at src/gen/CORE.setting:10159
  in method gist at src/gen/CORE.setting:893
  in method gist at src/gen/CORE.setting:10159
  in method gist at src/gen/CORE.setting:893
  in sub say at src/gen/CORE.setting:10963
  in block  at ./pb.pl:42

test.pl

use Grammar::Tracer;

grammar PB::Grammar {
    token TOP { ^ <proto> $ }
    token proto { [<message> | ';'] }
    token message { 'message' \s+ <ident> \s+ <message-body> }
    token ident { \w+ }
    token message-body { '{' (\s+)? <ident> (\s+)? '}' }
}

say PB::Grammar.parse('message fart {ass }')