perl6/doc/lib/X/Placeholder/Mainline

(title)  X::Placeholder::Mainline

    class X::Placeholder::Mainline is X::Placeholder::Block

Thrown when a placeholder variable is used in the mainline, i.e. outside of any explicit block. Example:

$^x

Produces the error message

    Cannot use placeholder parameter $^x in the mainline

Note that this error can also occur when you think something is a block, but it really is a L<< postcircumfix:<{ }> >>, for example

    my %h;
    say %h{ $^x };
    #     ^^^^^^^  not a block, so $^x is part of the mainline