So give the tar the one-way crypted version of the real filename. As in, roughly:

my $timestamp = timestamp_of( $innerfile );
my $tarname = crypt( $innerfile, $innerfile ) . ".tar";
$tarname = substr( $tarname, 2 );
system( 'tar $tarname cvf $innerfile' );
touch( $tar, $timestamp );
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The crypted filename will always be the same thing. So foo.txt will always create the same .tar file.

Also, you'll want to drop the first 2 chars of the crypted filename, because they are the salt, which in this case is the first two letters of the inner filename.

xoxo,
Andy

%_=split/;/,".;;n;u;e;ot;t;her;c; ".   #   Andy Lester
'Perl ;@; a;a;j;m;er;y;t;p;n;d;s;o;'.  #   http://petdance.com
"hack";print map delete$_{$_},split//,q<   andy@petdance.com   >