Often it is only the include and the exec command that people want. For one of my scripts I took only some lines out of CGI::SSI and send my templates through it.

$template =~ s/<!--#(include|exec) (\w+)="(.*)"\s*-->/ssi($1,$2,$3)/eg
+;
sub ssi{
    my($t,$n,$v) = @_;
    if($t eq 'exec'){
        if($n eq 'cgi'){
            $t = 'include';
            $n = 'virtual';
        }elsif($n eq 'cmd'){
            return `$v`
        }
    }
    if($t eq 'include'){
        if($n eq 'file' || ($n eq 'virtual' && $v=~/^\//)){
            my $file = $v=~/^\// ? $ENV{DOCUMENT_ROOT}.$v : $v;
            open(SSI, $file) or return "[SSI: $!]";
            my $re = join '', <SSI>;
            close(SSI);
            return $re;
        }elsif($n eq 'virtual'){
            eval "use LWP::Simple;1;";
            return "[SSI: $@]" if $@;
            return get($v);
        }
    }
}
[download]