One liner to get rid of warn output shown below, which occurs because $ENV{PWD} is not set.
$ENV{PWD} is likely wrong anyway as it does not update with chdir() which is done earlier.

 sudo rsnapshot -c rsnapshot.conf sync
 Use of uninitialized value $linux_lvm_oldpwd in concatenation (.) or string at /usr/bin/rsnapshot line 3720.
 Use of uninitialized value $linux_lvm_oldpwd in chdir at /usr/bin/rsnapshot line 3722.
 Use of chdir('') or chdir(undef) as chdir() is deprecated at /usr/bin/rsnapshot line 3722.
   Logical volume "backups-rsnapshot" successfully removed