sub replace_in_file
{
  my $file=shift;
  my $regexp=shift;
  my $replace=shift;
  my $txt=eval{local(@ARGV,$/)=($file); <>} or die("ERROR Load $file ($@)\n");
  $txt=~s/$regexp/&$replace/eg;
  open(my $fh, >, $file) or die("ERROR write $file ($!)")
  print $fh $txt;
  close($fh);
}

replace_in_file("${PATH}/etc/inittab",qr/remount,ro,noatime/,sub{return 'remount,rw,noatime'});