I am truly the world's dumbest human being.

I had

$ftp->put($file, $file) or die "Could not transfer $file to FTP server
+: $!\n";
[download]

thinking that I needed to specify the remote file name. When I tried the put_unique() method, it failed with an "invalid number of parameters error". My code now reads:

$ftp->put($file) or die "Could not transfer $file to FTP server: $@\n"
+;
[download]

and it works.

Maybe it is best that I don't breed...

Update: The Hash => \*STDERR option is really annoying... :-)

Thanks for all the help. I'll remember to RTFM and pay better attention to the problem next time.