my $line_id = 0;
LINE:
while (my $line = <$IN>) {
   $line_id++;
   next LINE if $line_id < $n;
   
   # now parse the interesting part of the file
   ...
}

##</code><code>##

{ local $. = 0; do {} while ($. < $n && <$IN>); }
while (my $line = <$IN>) {
  ...
}