You would have gotten the same result from the prompt. Your problem has nothing to do with Perl.

root:root_password@foo.bar.com:/my/remote/path_name
[download]

means file root_password@foo.bar.com:/my/remote/path_name on machine root. The path should be

root@foo.bar.com:/my/remote/path_name
[download]

You could feed the password to its STDIN (probably), but it would be better if you used public key authentication (so you don't need to specify a password at all).