When you -exec the perl script, it's done on the first file it finds, vaditerm.dt when it finds it, not when it has read the entire directory. Then when it continues, the next file it finds is vaditerm.dt.bak, which your -exec then operates on. Next is vaditerm.dt.bak.bak ...

One solution is to pass off the results of find to xargs; xargs then runs your perl program. If you want it to do one file at a time, there's an option to xargs to tell it so.
find . -type f | xargs perl -i.bak ...

Another solution is to tell find to ignore *.bak:
find . -type f \! -name '*.bak' -exec perl ...

The xargs version (if you process more than one file at a time) uses fewer process slots and will run faster, which may be important if you have a lot of files and/or your files are long. The xargs version will overwrite existing .bak files if they are physically in the directory after the primary files. Combining the two solutions, i.e.,
find . -type f \! -name '*.bak' | xargs perl ...
will certainly overwrite existing .bak files.