use strict;
use warnings;

my @array = ( 'a' .. 'd' );

generate(\&process, scalar(@array), [@array] );

sub process {
  print join(' ', @{shift()}), "\n";
}

sub generate {
  my( $code, $n, $array ) = @_;
  if( $n == 1 ) {
    $code->($array);
  }
  else {
    for( my($j,$i)=(0,1); $i <= $n; $i++ ) {
      generate( $code, $n - 1, $array );
      if( $n % 2 ) {
        $j = 1;
      }
      else {
        $j = $i;
      }
      @{$array}[$j-1,$n-1] = @{$array}[$n-1,$j-1];
    }
  }
}