Let's make 'where' work!

the token is found on line 905 of src/Perl6/Grammar.pm:

token variable_declarator {
    :my $*IN_DECL := 'variable';
    <variable>
    { $*IN_DECL := '' }
    [
        <.unsp>?
        $<shape>=[
        | '(' ~ ')' <signature>
        | '[' ~ ']' <semilist>
        | '{' ~ '}' <semilist>
        ]*
    ]?
    <trait>*
    <post_constraint>*  #this is added
    <default_value>?    #this is added as well
}