If you use two files, you could even avoid the memory requirement.

my $changed;

{
   open(my $fh_in,  '<', $fn_in ) or die;
   open(my $fh_out, '>', $fn_out) or die;

   while (<$fh_in>) {
      if (...[ need to delete this line ]...) {
         $changed = 1;
      } else {
         print $fh_out $_;
      }
      if (...[ need to insert a line after this line ]...) {
         print $fh_out ...;
         $changed = 1;
      }
   }

   close($fh_in);
   close($fh_out) or die;
}

if ($changed) {
   rename($fn_out, $fn_in) or die;
} else {
   # Preserve file's mtime.
   unlink($fn_out) or warn;
}
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