syntax.pl

grammar Syntax {
    token TOP { ^ <list> $ }

    token list { <expression>+ % <.space>+ }

    token expression {
          <match=.atom>
        | <match=.parens>
        | <match=.squares>
    }

    token atom { <.alpha>+ | <.punct>+ | <.digit>+ }

    token parens { '(' <list> ')' }

    token squares { <expression> '[' <list> ']' }
}

my $actions = class {
    method TOP($/) {
        make $<list>.ast
    }

    method list($/) {
        make $<expression>>>.ast
    }

    method expression($/) {
        make $<match>.ast
    }

    method atom($/) {
        make ~$/
    }

    method parens($/) {
        make $<list>.ast
    }

    method squares($/) {
        make $($<expression>.ast, $<list>.ast)
    }
};

say Syntax.parse($_, :$actions).ast.perl given 'f[a b c]';