I have not tried using B. But perldoc -m B shows that byteload_fh and byteload_string are in @EXPORT_OK. So you would need to do something like

use B qw(byteload_string);
[download]

to get that.

UPDATE
I am using Perl 5.005_03. But I doubt that this would have been changed in 5.6.x. In any case you can try that perldoc command and read the source-code for your version.