FWIW, I can replicate such a warning with this snippet

package ModPerl::ROOT::ModPerl::PerlRun::home_mcp_htdocs_cgi_2dbin_map
+perv2_2ejpg;

sub SEEK_CUR { 1 };

use IO::Seekable;  # by default exports the constant sub SEEK_CUR 
                   # (which clashes with the former definition)

__END__
Prototype mismatch: sub ModPerl::ROOT::ModPerl::PerlRun::home_mcp_htdo
+cs_cgi_2dbin_mapperv2_2ejpg::SEEK_CUR: none vs () at /usr/local/perl/
+5.10.0/lib/5.10.0/Exporter.pm line 66.
 at ./762841.pl line 6
[download]

but only with 5.10.0 — 5.8.8 interestingly does not produce that warning, except when I explicitly enable global warnings with -w  (which would explain why you didn't get it before).

To solve the issue, you probably need to remove the duplicate definition, or find whatever module is exporting it, too, and disable the export (in the above snippet that would be "use IO::Seekable ();")