foreach $line (@lines)
{
 if ($line =~m/^SY /)
 {
  $line =~s/^SY/CN/)
  if (next =~m/^SY /)
   {
    next =~s/^SY/  /;
   }
 }
}
# so if the first line starts 'SY' replace with 'CN', but if the next after the replacement does, then change SY on the next line for '  '.