Putting them into LIBS doesn't help.
How exactly did you specify it? I'm not 100% sure about Windows, but normally, putting
LIBS => "-L. -lopensc",
in the WriteMakefile() call in Makefile.PL should work just fine (IIRC, MakeMaker would expand the "." to an absolute path).
Did you see your -L/-l combo in the linking command issued when building the extension? Did you get a warning like "Note (probably harmless): No library found for -lopensc" (in which case the respective LIBS spec would be removed from the link arguments)?