This error-message is not saying that it can’t find Perl, nor anything about whether-or-not there is more than one Perl interpreter. It is saying that you have somewhere tried to use String; but String.pm cannot be found in any of the places which Perl knows to look for it ... the so-called @INC list (see perlfaq8 and other such places).
The “path to Perl” setting won’t be useful. What you need to specify is the include-path; the so-called PERL5LIB environment-variable content.
But first ... my Spidey Sense™ is going-off here. You said that you are just using a vendor-supplied control panel. Apparently, String is not a pre-installed module at your hosting company ... did you intend to install it; do you know how, if you did not? I ask this simply for clarification. Tell us more about where you’re coming from, and we can then more specifically help you.
No, upon re-reading, I see that you did install the modules. Let’s presume for simplicity that it worked. In this case, you can either arrange by some means to define the PERL5LIB environment variable, or, as I prefer to do, write a short “stub” script which issues a use lib statement and then invokes the “real” main-program. This causes the known-good search path to be added to the front of @INC so that the modules can subsequently be found.
use lib whatever... ;