I think that there is no error message because your $! is being reset by the unlink. Using Net::SMTP, you would replace various of the prints with methods from Net::SMTP:

(where ... == the rest of the line)

open (SENDMAIL ...)        ==>  $smtp=new Net::SMTP("localhost");
print SENDMAIL "HELO ..."  ==>  (implicit)
print SENDMAIL "MAIL ..."  ==>  $smtp->mail(...);
print SENDMAIL "RCPT ..."  ==>  $smtp->to(...);
print SENDMAIL "DATA"      ==>  $smtp->data();
print SENDMAIL "..."       ==>  $smtp->datasend(stuff);
print SENDMAIL "."         ==>  $smtp->dataend();
print SENDMAIL "QUIT"      ==>  $smtp->quit();
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Note that your could replace the data/datasend/dataend chunk with a single data(@msg) call, if you build the text of the message into @msg.