My understanding is that the problem is with the lack of restricted scope. Bareword filehandles are essentially
globalpackage scoped (thanks, LanX) so your RD could easily stomp on someone else's RD in a module which your code usesthe same package and vice-versa. These possibilities for action-at-a-distance are the downside.
Is there some official perl documentation about any such issue ?
Not that I'm aware of. There's an oblique mention in the FAQ which says "Since you want to be a good programmer, you probably want to use a lexical filehandle" but makes no mention of why good programmers want to use lexical filehandles.
Update: Just found this very interesting counterpoint where luminaries such as brian_d_foy and merlyn argue in favour of keeping them (almost a decade ago). The last point about strict is also interesting in the light of recent events.