Ensure you have a ~/.vim directory
Ensure you have a ~/.vim/ftdetect directory

Create the file ~/.vim/ftdetect/t.vim with this as the contents:

" definition file for testing scripts
au BufNewFile,BufRead *.t         set filetype=perl

This allows you to use perl syntax highlighting in vi for test scripts that use the ".t" file extension

How to add a '.t' filetype to vi's syntax highlighting

perl

Bash