The Perl 6 <?ws> token

token ws {
    [ \s+
    | ^^ [ \# \N*
         | <?pod_comment>
         ]
    | \# [ <?PGE::Text::bracketed: <({[>
         | \N*
         ]
    ]*
}

and thus

rule parameter_list 
  { <parameter>[ , <parameter>]* }

automatically parses whitespace and comments between parameters in a list.