Make every item in a list point to another item in a list, without pointing at itself. The middle part is needed for the special case where everyone has a "partner" except for the last person. In other words, in the list 1-2-3, if 1 points to 2 and 2 points to 1, then nobody can point to 3 so some swapping must be done. Inspired by the Kris Kringle node.
use strict;

my @one = (1..10); ## Sample set

my %point;
my ($x,$y);
my $total=0;
for $x (@one) {
  $total++;
  {
    ## Pick any list element but $x:
    redo while ($y = $one[rand @one]) eq $x;

    ## Special case: nobody left to point to
    if ($total==@one && !$point{$x}) { 
      $total = $point{$y}; ## Recycling $total
      $point{$y}=$x;
      $point{$x}=$total;
      last;
    }

    redo if $point{$y};
    $point{$y}=$x;
  }
}

print "$x => $point{$x}\n" while $x=$one[$x++];

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In reply to Swap-a-roo by turnstep

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