just once per second? performance is not going to be an issue: use the aproach that seems simpler for you.

I would go for...

sub first_line_from_file {
  my $fn = shift;
  open my $file, "<", $fn
    or die "unable to open file $fn";
  scalar <$file>
}
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update: added the scalar in front of <$file>. blazar, thanks for pointing it out.