my $pid = open $fh, "-|";

if ($pid) {  # parent here. Read from child.
    print "read from child: $_" while (<$fh>);
    wait;    # ...for the child to finish.
}
else {       # child here. STDOUT connected to parent $fh
    print while <STDIN>;
    exit;    # important, you don't want the child to execute
             # the statements after this block!
}
# parent continues ...