It sounds like a module that should have come with the script and should be in the same directory as the script, like the others mentioned. But since "." isn't in your @INC, perl won't find the module even if you do have it. Adding use lib '.' or use lib '/path/to' to the top of check_cit_nat.pl should help. Alternativel, PERL5LIB env var can also be used to specify where to look for modules.