Having trouble with IfStrEq from xstring

The result is “false”, because although the package claims to compare “strings”, it actually takes into consideration category codes.

The first character in your jobname is an l of category code 12, by general rule of TeX about generated strings of characters. However, you're comparing it with a standard category code 11 l.

You could do

deffn{jobname}
StrLeft{fn}{1}[firstChar]
IfStrEq{firstChar}{detokenize{l}}{true}{false}

because detokenize uses the same internal machinery that outputs characters with category code 12.