• @INC - even on windows - should have / as delimiter
• PAR is using an INC hook to load modules from memory
• the INC-hook will load those modules based on a hash look-up
• if backslashes are used in the string representations of paths they are interpreted as escapes

> what you are suggesting the OP could do differently here

• check @INC again
• check if the same problem is also happening on linux
• if that fails file a bug report with the crypt module

Anyway, while this was my intent, it seems like swl already identified the problem in the meantime: --> Re: PAR::Filter::Crypto Fails