You can avoid a temporary file by using a filehandle opened on a scalar:

my $msg = <STDIN>;
open(my $msg_fh,"<",\$msg)
  or die "Couldn't create scalar filehandle: $!\n";
my $file = "msg_".time.".txt";
# ...
$host->put($msg_fh,$file)
    or die "cannot put file: ", $host->message, $/;
[download]

Both of these versions may overwrite an existing file on the remote server, although it's not likely. I thought maybe using put_unique would help, but there doesn't seem to be a way to force the filename to end with .txt (at least with my FTP server).

Update: Fixed error in open statement, and lack of error checking that let that slip through. Also, this seems to miss the last byte of the string entered for some reason. I'm looking into it right now...

Update: The missing byte happens on my system with grinder's code, too. It looks like a bug in Net::FTP::A.

Update: This is CPAN RT Bug #25019.