use IPC::Open2;
local (*Reader, *Writer);
$pid = open2(\*Reader, \*Writer, "bc -l");
$sum = 2;
for (1 .. 5) {
    print Writer "$sum * $sum\n";
    chomp($sum = <Reader>);
}
close Writer;
close Reader;
waitpid($pid, 0);
print "sum is $sum\n";