All I would like to do is to remove the entire line which starts with

* @poseidon ...

print FILE map { s/.*\@poseidon$/ /g; $_; } @lines;

Instead of the $, you want to match the remainder of the line there (otherwise it won't match/be substituted), i.e.

print FILE map { s/.*\@poseidon.*/ /g; $_; } @lines;
                               ^^
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