Not a git expert, but a quick test seemed to show me that git reset --soft 2.12_002 will leave the changes from the newer versions in the index, I don't think that's what you want, because a commit afterwards would put those changes back into the repo.

If you want to begin your work with a commit that undoes all the changes since 2.12_002, the following seems to work, even directly on master.

git revert --no-commit 2.12_002..
git commit -m "Rewind to tag 2.12_002"
# continue work here
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Another approach might be to base the new branch on 2.12_002 and then later merge that branch back into master, clobbering all the changes in master at the same time auto-resolving all conflicting hunks by favoring the start-anew branch. (Update 2: Changed wording of the previous sentence after re-reading the git-merge docs. This is a significant difference because changes in the master branch that don't conflict will still be reflected into the merge result! Perhaps the "ours" instead of "recursive" merge strategy might be better for clobbering, untested. If in doubt, use the above solution instead!)

git checkout -b start-anew 2.12_002
# - make edits here -
git checkout master
git merge -s recursive -X theirs \
  -m 'Merge branch "start-anew" (based on 2.12_002)' \
  start-anew
git branch -d start-anew
git log --oneline --decorate --graph
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Update: Here's how I generated the fake repo for testing:

git init
echo -e "Hello, World\n---" >foo.txt
git add foo.txt
git commit -m 'Initial commit'
for VER in 001 002 003 004
do
    VER="2.12_$VER"
    echo -e "Added in $VER\n---" >>foo.txt
    git commit -am "Edit for $VER"
    git tag $VER
done
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echo -e "Added in 2.12_005\n---" >>foo.txt
git commit -am "Edit for 2.12_005"
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Update 2017-04-21: Just noticed this cross-posting to perl.module-authors. Also, it appears we're talking about File::Path version 2.12_004 (GitHub).