Yay sitecustomize.pl, no environment variable tweakage needed.

$ cat /var/local/perl/5.14.2/lib/site_perl/5.14.2/sitecustomize.pl
package AlwaysRun;
use Carp qw(:DEFAULT cluck);
cluck "$0 $$: I am begun at " . localtime();
1;

$ perl -E 'say "howdy"'
-e 16199: I am begun at Fri Mar  9 00:16:58 2012 at /var/local/perl/5.
+14.2/lib/site_perl/5.14.2/sitecustomize.pl line 3.
        require /var/local/perl/5.14.2/lib/site_perl/5.14.2/sitecustom
+ize.pl called at -e line 0
        main::BEGIN() called at /var/local/perl/5.14.2/lib/site_perl/5
+.14.2/sitecustomize.pl line 0
        eval {...} called at /var/local/perl/5.14.2/lib/site_perl/5.14
+.2/sitecustomize.pl line 0
howdy
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