Zaxo was on the money with readline() - all i needed was to get underneath the abstraction of the <FH> operator.

The spaceship operator (<>) is just a wrap around the readline function. You can get the same behaviour like this:

sub next_line {
  return scalar <FH>;
}
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Another remark, it might be worth setting $/ before using readline. That way you don't get unexpected behaviour if someone fiddled with it. So put something like this at the start of your sub:

  local $/ = "\n";
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-- Hofmator