$a = $b unless $error;

##</code><code>##

while ($a < $b) if not $error
 { $a++; }

##</code><code>##

$a++ while ! $error && $a < $b;

if ( ! $error ) {
   $a++ while $a < $b;
}

do { $a++ while $a < $b } if ! $error;

##</code><code>##

$a++ while not $error || $a >= $b;

$a = $b unless $error or $a >= $b;