if (open ...) {
  print error
} else {
  real code here
}
#EOF
[download]

is better written as

if (open ...) {
  print error
  exit(1);
}

real code here
[download]

Besides that, you might want to try splitting the script's processing stages into well-named subroutines. It will help you reduce the amount of variables in the current scope, too. And perhaps rid of the temporary file -- I'm sure MIME::Lite can attach a "file" from a string variable.