is X::Placeholder::Block
Thrown when a placeholder variable is used in the mainline, i.e. outside of any explicit block.
For example
;
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 ;say ;# ^^^^^^^ not a block, so $^x is part of the mainline