I just checked the perl source. I don't see any way of doing this for general references without patching perl. The good news is that's it's easy to patch, so if you're comfortable with recompiling your perl to debug your problem, it's an option.

For perl 5.8.5:

--- sv.c.org    2004-07-12 23:44:41.000000000 +0200
+++ sv.c    2004-10-03 16:54:08.000000000 +0200
@@ -3126,2 +3126,3 @@
         }
+        Perl_croak(aTHX_ "Stringification of reference disallowed");
         tsv = NEWSV(0,0);
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This still allows stringification of compiled regexen, or when overloading is in place. If you want to disallow stringification in those cases too (so, for all references), you can instead do:

--- sv.c.org    2004-07-12 23:44:41.000000000 +0200
+++ sv.c    2004-10-03 16:56:34.000000000 +0200
@@ -3001,2 +3001,3 @@
         SV* tmpstr;
+        Perl_croak(aTHX_ "Stringification of reference disallowed");
             if (SvAMAGIC(sv) && (tmpstr=AMG_CALLun(sv,string)) &&
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UPDATE: Just in case you're unfamiliar with applying a patch: unpack the perl 5.8.5 source, save one of the above snippets as "patchfile", execute (in the unpacked perl dir) the following command: patch < path/to/patchfile and then compile perl normally