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2 of 2 added 12 characters in body

Perl: 34 characters

$_="He12o wor3d!

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.)