When saving a fixed amount of incoming data and throwing it away most of the time as new data comes in, (eg, saving n preceding lines while(<>) processing files), this little gadget can be twice as fast as shift/push.
update:The code to read the array is simplified thanks to fundflow.
I originally posted this just as a novelty. But it seems to have turned out to be an interesting demonstration of the speed-up that can come from instruction locality (see below).
$sz ||= 3;   # user parameter

$top = $sz - 1;
@next = ( 1 .. $top, 0 );
$tl = $top;
@a = ();

#  . . .

$a[ $tl = $next[$tl] ] = $val;       # 'push' zero or more times

#  . . .

@all = @a[ $tl + 1 .. $#a, 0 .. $tl ];
$tl = $top;
@a = ();

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In reply to Circular buffer instead of shift/push by petral

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