You can eliminate the manifest temporary by:

($p->[$_], $p->[$swap]) = ($p->[$swap], $p->[$_]);
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Note though that it is slower:

use strict;
use warnings;
use Benchmark qw(cmpthese);

my @arr = 1..2;

cmpthese (-1,
            {
            temp => sub {my $temp = $arr[0]; $arr[0] = $arr[1]; $arr[1
+] = $temp;},
            swap => sub {($arr[0], $arr[1]) = ($arr[1], $arr[0]);},
            }
         );

          Rate swap temp
swap 1381331/s   -- -43%
temp 2425611/s  76%   --
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