That's a suggestion ("you can") for the case you expect new entries to come.

But that's not a problem with perltidy, cause cascades (as demonstrated) would be fixed automatically.

And I doubt that breaking simple ternaries would be appreciated by the majority.

It might interest you that tidy's output of a simple ternaries depends on existing linebreaks: (see -bot option)¹

lanx@lanx-1005HA:~$ cat /tmp/tst.pl

my $var1 =   $condition   ? $val_for_true 
:  $val_for_none ; #own linebreak

my $var2 =   $condition   ? $val_for_true :  $val_for_none ;

lanx@lanx-1005HA:~$ perltidy -pbp  /tmp/tst.pl

my $var1
    = $condition
    ? $val_for_true
    : $val_for_none;    #own linebreak

my $var2 = $condition ? $val_for_true : $val_for_none;
[download]

Since the documentation mentions ternaries only once for -bot I doubt your goal can be achieved! (i.e. without patching the code)

update

¹) i.a.W if you expect the ternary to be cascaded in the future then enter a linebreak manually.