open my $INPUT, '<', \"my input\n";
*ARGV = $INPUT;
print <>;
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That's not quite true:

% echo This is from STDIN|perl -e'print for <>'
This is from STDIN

% echo This is from ARGV > input1
% echo This is also from ARGV > input2
% perl -e'print for <>' input1 input2 
This is from ARGV
This is also from ARGV
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The <> operator looks first at at @ARGV and iterates line by line through each file specified as arguments to the script. But if there are no arguments (@ARGV is empty) then it falls back to reading from STDIN. This behaviour is documented in perlop, already linked to by the OP.

The diamond operator reads from ARGV, not STDIN.

That's not quite true

choroba is correct though, in that <> is always equivalent to <ARGV>, as is described in the very documentation you referenced. The magical behavior you describe is not that of the <> operator in general, it is that of the ARGV filehandle, which modifies @ARGV in order for STDIN to be read automatically via the ARGV handle.

Reworded for clarity, shortly after posting.

<> always reads from ARGV. It's just that ARGV might in turn read from STDIN. Whether it does or not depends on @ARGV.

in this case, the input from <> comes from "standard input". If this were exactly the same thing as "STDIN"

As per perlop and choroba's reply, <> is the same as <ARGV>.

Is there a way to override the "null filehandle" ?

That depends a bit on what you want to accomplish, could you describe that?* You've already gotten some replies on how to do this for a couple of cases, and I just wanted to add that messing with ARGV even more is possible, as I did in my module Tie::Handle::Argv. However, because of all of the trouble I had writing and testing that module, I can say that this is a pretty complicated thing to do, and I would not mess with <> if it can be avoided. Therefore your sample code makes me a bit nervous that you might be trying to do something that's not really advisable.

* So for example, if you just want to mock <> for testing, it's better to do that by calling your script from another script and feeding it stuff to its STDIN, which can be done reliably, or just by placing test files (e.g. generated via File::Temp) in its @ARGV (Update: for example, you can localize *ARGV - yes, the whole typeglob).

Like this ?

#!/usr/bin/perl

use strict; # https://www.perlmonks.org/?node_id=11154231
use warnings;
use autodie;

close STDIN;
open STDIN, '<', \"my input\n";
print <>;
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> Here's how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is empty, $ARGV[0] is set to "-", which when opened gives you standard input.

So what's happening is that STDIN is re-opened.

Did you try @ARGV = ($INPUT); instead?

update

just tested, doesn't work, this will try to use $INPUT as filename not handle...

From the proposed alternatives, I like choroba's best with *ARGV = \$INPUT

Had I known all your answers, I probably would have chosen to close and reopen STDIN because it best describes my intention.