how to change a string ,such as add a number after some letters

It's not working because it doesn't know where the first variable stops, and is trying to incorporate the 2 as part of the variable name.

Try this instead:

HTTP::redirect "http://${domainfirst}2${domainleft}[HTTP::uri]"

Here's an example in tclsh:

% set h1 www.srwd      
www.srwd
% set h2 9.com
9.com
% puts $h1$h2
www.srwd9.com
% puts $h12$h2
can't read "h12": no such variable
% puts ${h1}2${h2}
www.srwd29.com
% 

Colin