To disable the warning for those two variable:

# Prevent "used only once" warning for these variables.
$Mail::SendMail::error = $Mail::SendMail::error;
$Mail::SendMail::log   = $Mail::SendMail::log;
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To disable the warning for all variables:

no warnings 'once';
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Update: There actually is a typo! $Mail::SendMail::... should be $Mail::Sendmail::....