open(Source, "<".$dir.$tarif_file_source);
open(Dest,">>".$dir.$tarif_file_dest);
while (<Source>)
   {
   $data = $_;
   if ($data =~/<costin cost=/)
     {
     @elem = split('"', $data);
     $elem[1] = &Modif_cost($elem[1]);
     $data = join('"', @elem);
     }
   print Dest $data;
   }