Re: No child processes - system limit? shows a way to recreate the problem which it attributes to: Normally, you'd get this error (ECHILD) if you wait for a child, but there is no child

Also, from the source of IPC::System::Simple (sub capturex() called by capture()):

        # This next line also does an implicit fork.
        my $pid = open(my $pipe, '-|');  ## no critic
 
        if (not defined $pid) {
                croak sprintf(FAIL_START, $command, $!);
        } elsif (not $pid) {
                # Child process, execs command.
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So there is an implicit fork there expectedly. Additional to all your other forks.

So I would follow dave_the_m's suggestion Re: "No child processes" problem with fork() call and check if fork() does succeed before waiting for that child. fork() may fail when a maximum number of children has been reached according to the OS.