I think you'd pass it as an argument to configure ...

In perl5260delta.pod we find:

 "PERL_USE_UNSAFE_INC"

        There is a new environment variable recognised by the perl
        interpreter. If this variable has the value 1 when the perl
        interpreter starts up, then "." will be automatically appended
+ to
        @INC (except under tainting).

        This allows you restore the old perl interpreter behaviour on 
+a
        case-by-case basis. But note that this is intended to be a tem
+porary
        crutch, and this feature will likely be removed in some future
+ perl
        version. It is currently set by the "cpan" utility and
        "Test::Harness" to ease installation of CPAN modules which hav
+e not
        been updated to handle the lack of dot. Once again, don't use 
+this
        unless you are sure that this will not reintroduce any securit
+y
        concerns.
[download]

From that, I gather that you don't actually configure perl to have "." in @INC, but you instead set the environment variable PERL_USE_UNSAFE_INC to 1 in order obtain the "unsafe" @INC.

OTOH, we have in perlrun.pod:

    PERL_USE_UNSAFE_INC
                If perl has been configured to not have the current
                directory in @INC by default, this variable can be set
+ to
                "1" to reinstate it. It's primarily intended for use w
+hile
                building and testing modules that have not been update
+d to
                deal with "." not being in @INC and should not be set 
+in the
                environment for day-to-day use.
[download]

And this suggests that it is possible to configure perl such that @INC is "unsafe" by default.
I, too, would guess that would be done as Fletch proposed.

But I couldn't locate any definitive documentation on the matter.

UPDATE: I've just received word from the p5p list that the correct configure arg to use is -Udefault_inc_excludes_dot and that this is documented in the INSTALL file (which is located in the top level directory of the perl source distro).

Cheers,
Rob

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