As per The MDN page on perspective, I added a mixin and amended the include file.
Add a mixin for controlling the perspective for 3D transforms.
Remove this. No need to import it twice.
Thanks for the pull request. We should also create a mixin for the perspective-origin property: https://developer.mozilla.org/en-US/docs/CSS/perspective-origin
This has been integrated fully into Bourbon v3.0. Thanks @unaffectedscorn for the pull request!