The entire process will be forked, but both forks will continue running from the point the process forked. And since the data is also forked (copied), there's no problem. You'll get something like: 
foreach my $node_name (@nodes) {
    my $port_hashref = Mod1->sub1($node_name);
    my $pid = fork;
    die "Unable to fork: $!\n" unless defined $pid;
    unless ($pid) {
        # Child process.
        Mod2->sub2($node_name, $port_hashref);
        exit;  # When done, child should terminate.
    }
}
1 while -1 != wait;  # Reap children.

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In reply to Re: Forking sub inside a module by JavaFan
in thread Forking sub inside a module by vbruno

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