Another solution. The input array is known to have an even number of elements. Just take pairs off of the top of @input, swap 'em, push 'em to a temp array, then return that array when done. No need for recursion, indexing or anything "tricky".

#!/usr/bin/perl -w
use strict;

my @input = (1,2,3,4,5,6,7,8);

print swap_pairs(@input); # prints: 21436587

sub swap_pairs
{
   my @swapped_pairs;
   while ( my ($x,$y) = splice(@_,0,2) )
   {
      push @swapped_pairs,($y,$x);
   }
   return @swapped_pairs;
}
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