** But isn't the SSI-Command a Perl-based thing too? **
While SSI is usually part of Perl's installation, Perl scripts are not usually parsed for SSI tags which is the responsibility of the server set up.
Your server's set up may be able to be changed to parse Perl/CGI script output for SSI tags but you'll have to check with your host's support to see if it is possible on your server. Otherwise, you'll have to use a Perl function in the other script to capture CalendarScript's output and include it.
JMO
Dan O.
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