This appears to be a bug in the module. If you append ' 25' and ' 26' to $s1 and $s2 respectively, the function works as expected. I would suggest that you follow L~R's advice and give String::LCSS_XS a try, as it's 4 years more current.
I would hope that it is actually returning a space in front of the value that you said that you were expecting. Though, the documentation for String::LCSS is so minimal that I'd be wary of making any assumptions at all about what it does without first digesting the source code.