That won't work as-is because both BEGIN{} blocks will be executed before the if statement is even compiled.

If performance or memory is a consideration, unless you are using %Config for other things as well, you would be better off using $^O to test for the OS. It contain sthe same info as $Config{'osname'} but without the overhead of loading Config.pm.

Also, why are you putting a single scalar into an array and then unshifting the array?

This ought to come pretty close to what you are trying to do I think. (NB:untested)

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';
}
[download]

Though that is probably better coded as

(Also partially untested)

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