You'll need to rebuild perl (and libperl if it was built separately). Here's an example snippet of the patch used by a distro i work with:

--- perl.c.orig 2005-07-26 13:04:54.000000000 -0400
+++ perl.c      2005-07-26 13:05:05.000000000 -0400
@@ -4397,9 +4397,9 @@ S_init_perllib(pTHX)
     incpush(APPLLIB_EXP, TRUE, TRUE, TRUE);
 #endif

-#ifdef ARCHLIB_EXP
-    incpush(ARCHLIB_EXP, FALSE, FALSE, TRUE);
-#endif
+    /* for configuration where /usr is mounted ro (CPAN::Config, Net:
+:Config) */
+    incpush("/etc/perl", FALSE, FALSE, TRUE);
+
 #ifdef MACOS_TRADITIONAL
     {
        Stat_t tmpstatbuf;
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