#Perl: 35 characters

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

Sample run:

    bash-4.1# perl -e '$_="He12o wor3d!\n";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.)