Refer to Scalar value constructors.

B::Deparse shows how perl interprets this code:

perl -MO=Deparse -e 'my ${host_mail}="80.80.80.80";'
my $host_mail = '80.80.80.80';
-e syntax OK
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The curlies are allowed, but not required. I think is is more usual to use curlies only when referring to a variable, not when declaring it.