# Execute program and put it in the background.
system("program > out.txt &");

# Wait for file to be created otherwise this will fail.
sleep 3;

open(IFILE,"<out.txt") or die "Couldn't open file. $!\n";

# Keep reading until nothing more to read.
while (<IFILE>)
{
     # Do something with output.
     print "$_\n";
}
close(IFILE);