context, while is scalar, for is list, and readline reads the whole file in list context 
$ perl -MO=Deparse -e" ++$counts{$_} for <IN>; "
;
++$counts{$_} foreach (<IN>);
-e syntax OK

$ perl -MO=Deparse -e" ++$counts{$_} while <IN>; "
++$counts{$_} while defined($_ = <IN>);
-e syntax OK

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In reply to Re: Why is "for" much slower than "while"? by Anonymous Monk
in thread Why is "for" much slower than "while"? by di

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