One last point that might help a reader -- while the difference between $q->param() and $q->url_param() turned out to be interesting, I think the real issue with the CGI scoping came from the web server itself. I had 
<Directory /var/www/html/id/bin>
    SetHandler perl-script
    PerlResponseHandler ModPerl::Registry
    PerlOptions +ParseHeaders
    Options +ExecCGI
</Directory>

[download]
and that doesn't seem to like subroutines well. Switching to 
<Directory /var/www/html/id/bin>
    SetHandler perl-script
    PerlResponseHandler ModPerl::PerlRun
    PerlOptions +ParseHeaders
    Options +ExecCGI
</Directory>

[download]
and I haven't seen the problem since.

In reply to Re: CGI scoping question by WoodyWeaver
in thread CGI scoping question by WoodyWeaver

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