Well, I think your best bet would be to use the forking open.

Here's how you might do that (this is straight from Cookbook 16.10):

use IO::Handle;
if ($pid = open $child, "-|") {
    while (<$child>) {
        #look ma!  I'm processing!
    }
    close $child;
}
else {
    die "Couldn't fork: $!" unless defined $pid;
    STDOUT->autoflush(1);
    #exec program that wrties to STDOUT
    exit;
}
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