does use lib '/path/to/my/sandbox/lib'; append or prepend

It always prepends. So any stuff found in those extra directories won't be loaded from the default library directories.

It's in the docs, btw — one paragraph further down to the one you quoted :)

       Adding directories to @INC

       The parameters to "use lib" are added to the start of the perl 
+search path. Saying

           use lib LIST;

       is almost the same as saying

           BEGIN { unshift(@INC, LIST) }
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