Okay, I figured it was some peephole optimization thing. This is not what I would have predicted for deparse output, but it kind of makes sense. 
$ perl -MO=Deparse -e 'sub foo { 1 for 1 } foo'
sub foo {
    foreach $_ (1) {
        '???';
    }
}
foo ;
-e syntax OK

[download]
The '???' appears for other constants, with which it would generate "useless use of constant in void context" warnings. This warning is suppressed for a few common constants. So I guess the opcode optimizer just forgets WHAT constant was there.

--
[ e d @ h a l l e y . c c ]

In reply to Re^4: "last expression" quiz by halley
in thread "last expression" quiz by fxn

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