Why/how are these different?

[perl-diddler]

I have two snippets of code and I was wondering why they produced different results (P is like printf w/auto LF).

> perl -E '
use P;
for  ($_=3;$_-- >0;) { P "_=%s", $_ }'    
_=2
_=1
_=0

> perl -E '
use P;
P "_=%s", $_ for $_=3;$_-- >0; '
_=3

Note, I didn't say "for my $i...". That would make sense. But the behavior seems like that's what it did. However, a similar construct the reversed "if", doesn't act this way:

> perl -E '
use P;
$_=1;
P "_=$_" if --$_;
> P "got to end w/_=%s", $_;
> '
got to end w/_=0

#alternatively:
> perl -E '
use P;
$_=12;
P "_=$_" if --$_;
P "got to end w/_=%s", $_;
'
_=11
got to end w/_=11

This is in perl 5.16.1. Could someone explain why these look inconsistent to me? (i.e. "$_" value in the conditional statement isn't consistent with it's value in the test or outside the loop.

Thanks!
Curious one.

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FWIW -- the postfix 'while' seems to be consistent, giving the same results in regular vs. postfix form;