my $file = 'abc.txt';

{
    open my $in_fh, '<', $file or die "Cannot open input file: $!\n";
    flock $in_fh, LOCK_EX | LOCK_NB or die "Cannot obtain a lock on $file: $!\n";

    my $temp_out = File::Temp->new(TEMPLATE => "$0-$$-XXXXX");

    while (my $line = <$in_fh>) {
  while ( $line =~ /-abc\s(\d+)/g ) {
   my $first = $1;
   
   if (my ($tail) = $line =~ /^\.march_endSP)(.*)/) {
   print(".rs_$_$tail\n") for 1..$first;
} else {
   print($line);
}
  }
    
    close $in_fh or die "Failed to close $in_fh. Aborting. $!\n";

    $temp_out->flush;
    eval {
        move($temp_out->filename, $in_fh);
    } or do {
        warn "Failed to swap $temp_out into $in_fh: $!\n";

    };
}