Instead of "or die 'dead'", what do you get if you die "failed : $!\n"?

Beyond that, there's a few things to check. I believe that you can see if Mason caching is the issue by clearing out the mason cache/ directory. You can see if mason's "compilation" is to blame by clearing out the obj/ directory.

Do you have globals around that might cause an issue in a mod_perl environment?

You can always write a test script to hit your webserver and trigger the error. Making it easier to reproduce may help you find the cause faster.