In the case of Text::Csv you can certainly copy the source code provided you clearly attribute its original source and licensing terms on the copied file. - it is licensed under the same terms as perl. From the GPL
You may copy and distribute verbatim copies of the Program's source
code as you receive it, in any medium, provided that you conspicuously and
appropriately publish on each copy an appropriate copyright notice and
disclaimer of warranty; keep intact all the notices that refer to this
General Public License and to the absence of any warranty; and give any
other recipients of the Program a copy of this General Public License
along with the Program. You may charge a fee for the physical act of
transferring a copy.
However, I'd be cautious, however, generalizing that to all CPAN modules. There are quite a number of licensing models for CPAN modules. Some, for example, do not allow commercial use. So as a general rule, one should always check the licensing before copying, and sometimes even using a CPAN module.