This is documented in perlsyn:

The while and until modifiers have the usual "while loop" semantics (conditional evaluated first), except when applied to a do-BLOCK (or to the deprecated do-SUBROUTINE statement), in which case the block executes once before the conditional is evaluated. This is so that you can write loops like:
do {
    $line = <STDIN>;
    ...
    } until $line  eq ".\n";

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In reply to Re: do {$i++} until vs. $i++ until by Anonymous Monk
in thread do {$i++} until vs. $i++ until by jethro

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