We recently moved to perl 5.26 from 5.20 ( https://lab.nexedi.com/nexedi/slapos/commit/46124557a113a6743ffb9428aeaf6d2133fd44bb ), which seems to breaks the pt-digest.
The fix for percona tools was added in release 2.2.17 onwards and the patch for the same can be found here: https://github.com/percona/percona-toolkit/pull/73
This patch changes the 2.2.15 version to 2.2.17.
The changelog can be found here - https://www.percona.com/doc/percona-toolkit/LATEST/release_notes.html#v2-2-17-released-2016-03-07