Well leaving aside locking issues, and the point that the number 2 by itself is not exactly valid html, here is the above made even shorter.

perl -MCGI -pi -e 'END{print&CGI::header,$t}s/\d+/$t=$&+1/e' c
[download]

UPDATE
And, of course, there is always a shorter way:

perl -MCGI -pi -e'END{print&CGI::header,$t}$t=++$_' c
[download]