Personally, I don't accept input from (untrusted) users. But that approach certainly isn't feasible if you want to create a website that allows users to enter data. When I output stuff, I'd really like a way to specify the escaping in the templates like the Free Nodelet allows, by appending &, % etc..
Something that I'm thinking about from time to time would be a more typed version of Taint mode where you can "color" strings according to their provenience (user input, db input, etc.). You would also need to be able to color the filehandles and other output/system methods accordingly, and a HTML-colored output filehandle would either die fatally when it encounters input in the wrong color or convert the input by html-escaping it.
To make this idea feasible at all, concatenation with constant strings would need to bleed the color into the result and some translation rules would need to exist. I'm not sure where Perl has hooks for that. I believe Taint mode is implemented through magic, so maybe the colors could be implemented through the same magic, except that a colored string is both tainted ("lead paint") but in a special color.