I imagine you already know why is better to use lexical filehandles with 3 arguments (enforcing the scope, autoclose,...), but as you are asking about official docs I cannot find anything. Well generally speaking perl documentation is very eterogenous in style. At least.

> What's so wrong with doing open RD, '<', 'file.txt' ?

Nothing if you know what happens in this line and in the whole code you are writing.

perl -we "use strict; open FH, '>', 'text.txt'; open FH, '>', 'text.tx
+t';"

perl -we "use strict; open my $fh, '>', 'text.txt'; open my $fh, '>', 
+'text.txt';"
"my" variable $fh masks earlier declaration in same scope at -e line 1
+.
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I consider the second line less error prone, if you have thousand lines of code with many open FH statements the risk to forget to close one of them and mess the whole thing is high.

So it is a matter of (better) habits and nothing wrong per se. See also why-the-modern-perl-book-avoids-bareword-filehandles. And perlmaven.

More: if it is a matter of good habits I will enforce an exception also in the 2 arguments form of open What happens if you feed open my $in, $filename; with >/etc/passwd ? Infact 2 arguments form is dangeourous.

If it was my decision I will enhance strict pragma to deal with this:

use strict;

open FH, '<', 'filename'; # dies
open my $fh, 'filename';  # dies

no strict 'open';

open FH, '<', 'filename'; # now ok
open my $fh, 'filename';  # now ok
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L*