Okay, here's what I found:

$ perl -mO=Deparse -le 'print "$ARGV $_" for (<>)'

foreach $_ (<ARGV>) {
    print "$ARGV $_";
}
[download]

COMPARE:

$ perl -mO=Deparse -le 'print "$ARGV $_" while (<>)'

print "$ARGV $_" while defined($_ = <ARGV>);
[download]

Since $ARGV is set every time <ARGV> is evaluated, when you do it in a while loop, $ARGV is set correctly for each file in the loop. But using a for loop, you slurp <ARGV> in one shot, then iterate over $_, so $ARGV is set to the last file that was iterated over the last time <ARGV> was evaluated...

Hope that makes sense...

Update: I guess I just repeated what broquaint already said, but since I didn't understand what he wrote, I posted this, thinking I had something new to offer. Perhaps someone else will find it easier to understand, too...

--
3dan