open $h, "| program_A" or die ...;
foreach (@data_files) {
   print $h ...;
   print $h ...;
    ...
}
close($h);

##</code><code>##

foreach (@data_files) {
   open $h, "| program_A 2>& /dev/null" or die ...;
   print $h ...;
   print $h ...;
    ...
}
close($h);