Before building the email message, check all variables and perform appropriate actions if they aren't set or don't have suitable values. Those actions might be to call an error handling routine, output a warning message, supply a default value and so on.

You might also consider building the components of the message separately. For instance:

if ($email_400) {
    my $problem_description = $ENV{REDIRECT_URL}
        ? "A request was received for the file $ENV{REDIRECT_URL}, whi
+ch does not exist."
        : "A file request was received but no filename was supplied.";
    send_mail(...
    ...
    $problem_description
    ...
EOF
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