my $changed = 0;

foreach $line (@lines)
{
  if ($line =~m/^SY /)
  {
    if ( $changed == 0 )
      {
        $line    =~ s/^SY/CN/)
        $changed =  1;
      }
    else     
      {
        $line    =~ s/^SY/  /;
        $changed =  0;
      }
  }
  else
  { $changed = 0; }
}