I suggest a recursive approach. While this might not be the fastest one, it is easy to write:

sub vari { 
  my ($set,$num,$act)= @_; 
  my(%h,$rec,@s); 
  $h{$_}= 0 for @$set; 
  $rec= sub { 
    $_[0] or return $act->(@s); 
    $h{$_} or do {
      local $h{$_}= 1; 
      push @s, $_; 
      $rec->($_[0]-1); 
      pop @s; 
      } 
      for @$set; 
    }; 
  $rec->($num); 
  } 
vari [qw{a b c d e f g}], 3, sub 
  { print join(" ",@_), $/; };
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Update: changed code to be stable.