D:\tmp>perl -E"say '.' x rand(70) . '# test comment' for 1..3"
....................# test comment
...................................# test comment
.......................# test comment

D:\tmp>perl -E"say '.' x rand(70) . '# test comment' for 1..20" |perl -lpe " /(.*?)(#.*)/ and $_ = sprintf qq{%-68s  %s},$1,$2"
.....................                                                 # test comment
..............................................................        # test comment
.....................................................................  # test comment
....................................                                  # test comment
..........................................................            # test comment
...............................................                       # test comment
............................................................          # test comment
.............                                                         # test comment
...........................                                           # test comment
...............                                                       # test comment
..............................                                        # test comment
....................................                                  # test comment
...................................................                   # test comment
.................................                                     # test comment
...........................................................           # test comment
..........                                                            # test comment
.......................................................               # test comment
.................................................................     # test comment
..........................................                            # test comment
................................................................      # test comment