Swap the role of the parent and child (that is, let the parent do the heartbeating). Assuming you're using Unix (perhaps other OSses do the same, but I don't know them well enough), if a child process terminates, the parent receives a SIGCHLD. Then add to the parent:

$SIG{CHLD} = sub {exit};
[download]

Otherwise, you may be able to use a pipe between the parent and the child. The child should receive a SIGPIPE if it tries to read from the pipe after the parent died. But I haven't tried that in code.