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.
For example
$^x;
dies with
===SORRY!=== Cannot use placeholder parameter $^x outside of a sub or block
Note that this error can also occur when you think something is a block, but it really is a postcircumfix:<{ }>, for example
my %h; say %h{ $^x }; # ^^^^^^^ not a block, so $^x is part of the mainline