How would I enable my perl program to DD a file to all of those slots, ideally at the same time?

The "cheapest" way would be to do it like on the (bash/zsh/ksh/...) command line, i.e. by running the command in the background:

system("dd ... &");
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However, if you need to monitor the child processes, for example by waiting for them, you need their PIDs. In this case, you have two options: Use the special form of system, which has the digit 1 as the first parameter, i.e.

my $child_pid=system(1,'dd','first_parameter',....);
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or use fork and exec.