try executing the following from the command line:
prompt>perl -e 'use Benchmark;'
If your module isn't installed it'll give you an error "Can't locate Benchmark.pm in @INC (@INC contains blah/blah/blah)
Begin failed -- Compilation aborted at -e line 1.
Just a stupid trick executing code to see if a module is installed and if your perl interpreter knows how to find it. The special array @INC contains the pathway in which perl will search to find whatever modules you use and require. You could modify it at compile time with
use lib '/path/blah/blah';