my $Usage = "usage: volume root_dir subvol timeframe dateformat keep\n";
@ARGV == 6 or die $Usage;  # we need six arguments

my (
  $subvol_to_snapshot_arg, # this
  $snap_root_dir_arg,      # assigns all variables
  $yabsm_subvol_name_arg,  # in one go
  $timeframe_arg,          # and leaves room
  $date_format_arg,        # to comment
  $snaps_to_keep_arg,      # what they are about
) = @ARGV;