=begin pod

=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

=end pod