Thanks Ken,

I hadn't realise that MIME::Base64 was core!

What I had done was to SSL onto the server and type

perl -e "use MIME::Base64"
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This told me it couldn't find it which is why I tried to install it got the error that it needed access to the compiler. I did the same thing with Authen::SASL but that installed fine.

Having found it is core I have checked again and found that MIME::Base64 is there. I guess I must have put in a typo :(

So now I have a different issue to resolve...

Niether of these throw an error:

perl -e "use MIME::Base64"
perl -e "use Authen::SASL"
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Yet, when I try to use MIME::Lite in SMTP mode I get the error:

SMTP auth() command failed: No such file or directory
Need MIME::Base64 and Authen::SASL todo auth
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UPDATE:
Solved the error - it's a quirk of cPanel that I need to use cPanelUserConfig; to add all the correct paths to @INC when I've installed modules. Authen::SASL in this case.