The second argument is not undef. The double quotes 
"$ENV{'HTTP_HOSTS'} "

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says that you are trying to concatanate an undef value with, in this case, a space. That is what Perl is complaining about. Remove the quotes and the result is 
HTML

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nit: You don't need the quotes around the hash key either.

In reply to Re: Why does an undef argument in a join produce this warning? by wfsp
in thread Why does an undef argument in a join produce this warning? by Anonymous Monk

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