Accompanied with related utilities.
All programs give usage information when run without arguments.
The program custar.pl, when given same arguments where mtime
is absolute time, creates exactly same archive when same content
is available in file system.
The program custar.sh does the same with almost identical arguments,
and uses (new enough) GNU tar to do the archiving. In many cases
the created archive is exactly same as with custar.pl.
I bet other tar archivers (e.g. bsdtar) can do the same, I just haven't
tried such a case. (custar.sh is mainly used as output comparison tool)
custar.pl is BSD (2-clause) licensed (pure) perl program, which can be (easily?) edited should a need arise.
The tar format these utilities know, POSIX 1003.1-1988 ("Unix Standard TAR") has some limitations (link targets max len 100 octets, file/dir pathname max len 255 octets (when subdir length matches 155 octets)) -- usually these limits are far enough but be prepared...
Lists ustar archive contents, the subset of file information chosen
by flags argument.
List differences of two ustar archives. This has reasonably good usage,
may help use of custarlist.pl too (in addition to reading the code).
Keep, remove, and rename files from/in a ustar archive, writing modified archive to stdout (failing if stdout (fd 1) is referring to a tty).