Use $^X rather than "perl"

Thanks for your work on this module.
This short pull request changes the hardcoded "perl" to `$^X`:

https://rt.cpan.org/Public/Bug/Display.html?id=95263
Use $^X rather than "perl"

Fix interpolation bug of $cfgexp I accidentally introduced.

This short pull request changes the hardcoded "perl" to $^X:

https://rt.cpan.org/Public/Bug/Display.html?id=95263