The documentation for these modules is misleading. GD::Barcode::UPCE has $text $oGdBar->{$text} while GD::Barcode::Code39 has $text $oGdBar->{text}. Checking the source, $oGdBar->{text} would be correct for both modules.

The message "Can't locate object method "some_method" via package "Some::Module" (perhaps you forgot to load "Some::Module"?) at ..." generally indicates someone wrote:

$some_object->some_method(); # where $some_object is an instance of So
+me::Module
[download]

but should have written:

use Some::Module;
...
$some_object->some_method();
[download]

I can't see a reference to the method Small() in your code but I can see two in the source code for GD::Barcode::Code39 in the plot() method.

If I were you, I'd raise a bug report at this point. In case you don't know, just go to GD::Barcode::Code39 and follow the Report a bug link in the CPAN RT box (near the top right-hand corner in my browser).