Well, you could go for JSP/ASP style constructs like <% %> and <%= %> which at least are fairly unambiguous but a bit clunky. Without using special unicode characters it's hard to see how you could reduce the construct to a single (pair of) character(s) - update: if you're using Perl, that is. In Lisp, it could probably be done a lot easier.

There are a few modules on CPAN that already parse this kind of syntax. Or take a look at what other template modules on CPAN use.

Update2: depending on your exact needs, you could also get away with things like sub { $1 + $2 } but that more or less implies you're not parsing whatever is in $1 and $2 at compile time.