my $returnVal=threads->create(&threadFunc)

does not appear to be correct, you are calling 'threadFunc' and using its return value in threads->create as the function name/ref. This is probably why $returnVal is undef, although I would have thought that you would check that in your code. To quote the threads documentation:

FUNCTION may either be the name of a function, an anonymous subroutine
+, or a code ref.

    my $thr = threads->create('func_name', ...);
        # or
    my $thr = threads->create(sub { ... }, ...);
        # or
    my $thr = threads->create(\&func, ...);
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