For my script I am now using local to protect the system from being overridden under mod_perl. I don't really know if it's the right way, it's only the result of trial and failure.

    #!/usr/bin/perl
    use strict;
    use HSP;
    if ($ENV{MOD_PERL}) {
        my @keys = keys %HSP::;
        my $do = '';
        foreach(@keys){ $do .= "local *HSP::$_ = *HSP::$_;"; }
        $do .= 'HSP::handle();';
        eval $do;
    } else {
        HSP::handle();
    }
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In fact, Toolkit, which you mentioned, has an option for variable interpolation, I just found that line that says it in the documentation. It's quite helpful when coding a template with an ASP capable editor. The code blocks appear in clickable boxes and the variables can be entered in the visible HTML. Only TT2 does use square brackets [% %] by default which should be changed to <% %> then.
However just for my personal pleasure I would like to put perl within the brackets so I can have everything in one place. I have long enough put everything in the cgi-bin and started with #!/usr/bin/perl, I need a change!! Playing with this saves me from switching to php.