$ for prsr in prism parse.y
do                                                                           
    ruby --parser=$prsr -vce '[:out].each { p _1; [:in].each { p _1 } }'
done

ruby 3.4.0dev (2024-12-05T16:28:02Z master d8dbc2dcba) +PRISM [arm64-darwin24]
Syntax OK
ruby 3.4.0dev (2024-12-05T16:28:02Z master d8dbc2dcba) [arm64-darwin24]
-e:1: numbered parameter is already used in
-e:1: outer block here
[:out].each { p _1; [:in].each { p _1 } }
ruby: compile error (SyntaxError)