@echo off
setlocal enabledelayedexpansion
 
set "dirvar=%~dpn0.%~nx1"
mkdir "%dirvar%"
 
for /f "usebackq delims=" %%a in ("%1") do (
	set /A x+=1
	echo Linea !x!
	echo %%a>"%dirvar%\!x!"
)
 
Pause&Exit

You can't break a limitation of the OS, you can't break the 255 chars path in Windows, and you can't break the CMD interpreter lenght limitation, simply as that!

Sorry but you can't store that line into a var, no way, don't exist any magic, computers are logical.

But it's not the end of the world, you can do it so easy in any other lenguage, I recommend you Ruby or python (Ruby for that), it's an easy job, open a file, store the content into a var, and then do what you want, don't need any experience for that, if you need a example just comment this.