#Perl: 34 characters

<!-- language: lang-perl -->

    $_="He12o wor3d!
    ";s{\d}{l}g;print

Sample run:

    bash-4.1# perl -e '$_="He12o wor3d!
    > ";s{\d}{l}g;print'
    Hello world!

(Not a big deal. Posted just to use at least once in my life `s///` with those fancy delimiters.)