With respect to Perl::Tidy, it is quite obvious that a tidy file is one that has gone through perltidy and which does not change if reformatted again.
$ perltidy tidy_file.pl
$ diff tidy_file.pl tidy_file.pl.tdy
# zero, no difference
Perl::Tidy is a work in progress which is constantly improving. And improving means changing. Those changes may include the fixes of some behaviors considered non-ideal or even flawed. So tidy files under a
Perl::Tidy version may not be tidy under another version. So this concept is a little fragile (or needs a more precise wording).
Even Test::PerlTidy (which intends to capture that tidy-ness concept in the form of an automated test) was caught by this detail, since a new release of
Perl::Tidy got out. Edmund von der Burg and me are working on a reasonable approach to this issue. Wait for a new release of