Run it through B::Deparse to see how perl sees it:

$ perl -MO=Deparse -e '$foo += $_ for @e;'
;
foreach $_ (@e) {
    $foo += $_;
}
-e syntax OK
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As the output shows, that line is exactly the same as if you had used foreach $_ (@e) { . . .. Taking the for to the end of the line just allows you to cram it into one statement, for better or worse.