The problem you are having with taint here is exactly the same problem that you had before you learned how to untaint RealBin. Any time to want to use an external variable, like the $FindBin::RealBin or an $ENV{...} environment variable to access the filesystem, you have to untaint it.

I used the command line on my cPanel-based webhost to prove the point: running DOCUMENT_ROOT=/home1/pryrtcom/public_html perl -T sscce-t.pl :

DOCUMENT ROOT = /home1/pryrtcom/public_html
inside eval block to avoid dying on the tainted environment variable
Outside of eval block
ROOT = /home1/pryrtcom/public_html
inside second eval block to avoid dying on the tainted environment var
+iable
eval 2 didn't die if this prints
it did not die because I untainted /home1/pryrtcom/public_html.
[download]

source:

Learn the lesson of taint: essentially anytime you use a variable that comes from the outside world, you have to untaint it before using it to access the filesystem. (Second lesson: taint error messages are unhelpful. The problem is not with IO::File, but with using a tainted variable to try to access the filesystem.)