All I can think of is that one of your libraries is also manipulating @INC. My perl 5.10.1 is behaving well: use lib 'e1'; use lib 'e2'; print join "\n", at INC; e2 e1 C:/aPerl/site/lib C:/aPerl/lib use lib 'e1'; use lib 'e2'; print join "\n", at INC; e2 e1 C:/aPerl/site/lib C:/aPerl/lib -y