Rather than telling you how to find the pids and kill them, I'm choosing to describe a better way to "cat" the device, that requires no extra pids.

open my $in, "/dev/urandom" or die "couldn't open random: $!";
open my $out, ">a_file.bin" or die "couldn't open output file: $!";
eval {
    alarm 1;
    my $buf;
    read $in, $buf, 1024 or die "error reading: $!";
    my $orig = select $out; local $| = 1;
    select $orig;
    print $out $buf;
};
close $in;
close $out;
[download]

That was from memory, so if I got something wrong, don't sue me or anything. I also think you can make it a lot better than what I put above, but I also think all the right stuff is in there so you can cook from it with little trouble.