Here is a program that does the swapping with one temp only:

use strict;
use warnings;

my @buffer = qw[X0 X1 X2 X3 X4 X5 X6 X7 X8 X9 Y0 Y1 Y2 Y3 Y4 Y5 Y6 Y7]
+;

sub swapsamesize {
  my( $s1, $s2, $n, $buf ) = @_;
  ($$buf[$s1+$_], $$buf[$s2+$_]) = ($$buf[$s2+$_], $$buf[$s1+$_]) for 
+0..$n-1; # too lazy to use $tmp
}

sub swapbuffers {
  my( $n1, $buf ) = @_;
  my $ntotal = @$buf;
  my $n2 = $ntotal - $n1;
  my $d = $n1 - $n2;
  print "$n1 $n2 $ntotal $d\n";
  print "@$buf\n";
  my( $s1, $s2, $n ) = ( 0, $n1, $n2 );
  swapsamesize( $s1, $s2, $n, $buf );
  print "@$buf\n";
  ( $s1, $s2, $n ) = ( $n2, $ntotal - $d, $d );
  swapsamesize( $s1, $s2, $n, $buf );
  print "@$buf\n";
  ( $s1, $s2, $n ) = ( $n2, $ntotal - 2 * $d, $d );
  swapsamesize( $s1, $s2, $n, $buf );
  print "@$buf\n";
  ( $s1, $s2, $n ) = ( $n2, $ntotal - 3 * $d, $d );
  swapsamesize( $s1, $s2, $n, $buf );
  print "@$buf\n";
  ( $s1, $s2, $n ) = ( $n2, $ntotal - 4 * $d, $d );
  swapsamesize( $s1, $s2, $n, $buf );
  print "@$buf\n";
}

swapbuffers 10, \@buffer;
[download]

Of course, the repeated calls to the utility function swapsamesize should be done in a loop until all is done. It also only works if the overlap when swapping (i.e. n1 - n2) is smaller than the rest of the smaller half. I think the logic can be adapted...

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