Migrating from git to Perforce

Have you checked out the tool "tailor"? It's build for synchronizing different VCS:es. It's supposed to have Perforce-support.

As a side-note, my first reaction would be to seriously question the decision, but I guess you've already done that.

The option of "just throw away the old history" is not as bad as it sounds: you can just keep your git repo alongside it forever, in case anyone needs to dig through the old stuff. Unfortunately, there is just no way to represent git's complex view of history in old-style linear systems like svn and p4.

The main reason to look back into old history is for things like 'git annotate' (I assume p4 has a similar tool). If that's all you want, then maybe what you really want to do is squash all your merge commits down to only one of their parents (so they look like a single commit instead of a merge). That's more like what svn and p4 would have recorded in their own history model, where merges just look like a single commit in the linear stream. You can probably do this with git-filter-branch or the like. Of course, this would lose all the history that happened on sub-branches... but p4 users are used to not having that information.