tr[a-zA-Z][m-za-nM-ZA-N];

That isn't a rot13, it's a mistake.

Rot13 is tr[a-zA-Z][n-za-mN-ZA-M];

And here it is as a one liner that Rot13's all files specified on the command line, and saves their original as filename.old:

perl -pi.old -e "tr/a-zA-Z/n-za-mN-ZA-M/" filename