A lot of people have said this already, but CGI.pm is XHTML compliant, yes. As of 2.69 it defaults to XHTML.

I want to contribute the secret to getting it not to use XHTML, which I had to search for in the POD recently:

 use CGI qw(-no_xhtml);
[download]

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($_='kkvvttuubbooppuuiiffssqqffssmmiibbddllffss')
 =~y~b-v~a-z~s;                             print
[download]