$argact = $ARGV[0] or die "No number given\n";
open INPUT, 'one.txt' or die $!;
open NEWFILE, '>', 'output' or die $!;
while (<INPUT>){
  if(/^Authentication\s+number\s+is/) {
    my ($old_num) = /(\d+)/;
    if ($argact != $oldnum) {
      $changeflag = 1;
      s/\d+/$argact/;
    }
  }
  print NEWFILE;
}