I just did this, and the "how to" was in the "Perl Cookbook" on pages 451-452. I recommend you take a look at the description, but the explanation is to but the test in a BEGIN block:

BEGIN {
        unless ( eval "use Fictitious::Module;, 1" ) {
            warn "couldn't use Fictitious::Module: $@";
      }
}
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I hope I typed it correctly, as the book explains why it has to be exactly this way.

Good Luck