BEGIN{
# UPDATE: Corrected typo pointed out my [Abigail-II] below.
    unshift @INC, $^O =~ m[^(?:MSWin32|solaris)$]i
                ? '/foo/custom/perl/lib/Custom_mods5.6.1'
                : '/foo/custom/perl/lib/Custom_mods5.005_03';
}

##</code><code>##

BEGIN{
    use lib $^O =~ m[^(?:MSWin32|solaris)$]i
                ? '/foo/custom/perl/lib/Custom_mods5.6.1'
                : '/foo/custom/perl/lib/Custom_mods5.005_';
}