First off, the words, "code I found on the net" send shivers up the spine. My advice: look at the CPAN modules found here.

Having said that, let's take a look at a visually enhanced version of what you found.

sub print_html_page_w_ssi {
   # grab the .html filename passed as a parameter
   $a = shift;

   # zap any backslashes in the name
   # (note: we seem to be on a windows machine)
   $a =~ s/\\//g;

   # spiff up the filename, if needed
   $a = glob($a);

   # grab its contents (i.e. the html code itself) in $_
   $_ = `cat $a`;

   # print out the http header
   print "Content-type: text/html\n\n";

   # process any ssi tags in $_ one-by-one
   while ( /<!--#exec cgi="([^"]*)"-->/ ) {
      # now the name of the ssi file is in $1
      # run that file and grab its output in $a
      $a = `$1`;

      # put a <br> with each line terminator
      $a =~ s/\n/<br>\n/g;

      # and substitute that output for 
      # the ssi tag where it occurs in $_
      s/<!--#exec cgi="([^"]*)"-->/$a/;
   }
   # print the resulting page (which is still
   # in $_) to the waiting web client
   print;
}
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