Shouldn't this be fixed with 3-argument open?

Fixed implies broken. This feature is there by design, and predates perl5. I find it useful.

I really like the magic-diamond for quick one-liners, but this just sounds all the security/robustness alarm bells.

One-liners are one-liners. They are there for convenience. Writing secure and/or robust programs means you're going to put in more effort than a one-liner.

One of perl mottos is to make "easy things easy, and hard things possible". Magic open is a form of easy. Writing secure and robust programs is a hard thing. For that, you taint your command line arguments, and use 3-arg open. Note that a simple -T flag prevents your example from doing any harm:

$ perl -TwE '$ENV{PATH} = "/bin"; while (<>) {say}' '/bin/rm * |'
Insecure dependency in piped open while running with -T switch at -e l
+ine 1.
$
[download]

Replacing magic open with 3-arg open means easy things are not so easy any more.

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