I have not used IPC::ShareLite, and am at home without a Unix machine to test, but after looking through the module, it appears the invalid arg message is caused by a return of undef from read_shre (the XS function).

as i said, i have not tested this, but here is what i am seeing :
1. fetch (SharLite.pm) calls read_share
2. read_share (ShareLite.xs) calls system function, and return string, or undef if string length is <= 0
3. fetch (ShareLite.pm) error if return from read_share is undef

Based on this, i would guess that if fetch() is being called prior to adding data to the share, it could possibly error ... but, thats just a guess.