>I would like to use a SSI ON my template to include my standard header
You cannot do this. Perl scripts are not html documents - the server won't parse them for SSI commands.
Instead, what you need to do is check the documentation for how to include a file into the CalendarScript templates - it's the same format as regular SSI's - and edit the html files in the template folder to include this command. Then CalendarScript will automatically find the file and include it in the output, much like an SSI would work.
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Scott
CalendarScript.com