open OUT,'|program' or die $!;
open IN,'file.txt' or die $!;
#if you don't need to process each line...I don't see why not printing all the lines at once like:
print OUT <IN>;
#but if you must print one line at a time:
while(<IN>){
#do whatever you want with the line.
print OUT $_;
}
close OUT; close IN;