To elaborate on the above response, I think the culprit is:

$sender =~ s/@/\\@/;
[download]

(Note that the s/@/\@/ in the above response do not actually change anything.) In general there shouldn't be any need to escape at-signs in your email addresses. If you do, sendmail will add @yourdomain.com to fully qualify the address.

In this line:

print MAIL "To: ???\@gmail.com\n";
[download]

you need to escape the @ only because it is in a double-quoted string and you don't want it to be interpreted as array interpolation. If you used a single-quoted string you wouldn't need the backslash. However, in previous line:

print MAIL "From: $sender\n";
[download]

there is no need to escape any at-signs in $sender - its value will be inserted without any further interpretation.