Just another example of a conversion (using the boilerplate recipe). I'm posting mostly so it will be here to refer back to if I revisit the topic. It's interestingly different from the other examples, and the recipe still works well (although I get a curious Use of uninitialized value in array element warning if I don't turn those warnings off (which I do, below)).

What I'm generating is permutations, like Algorithm::Loops NextPermute.

#!perl
use strict;
use warnings;

sub no_repeat_combos {
    return [@_] unless @_ > 1;
    # Find the first occurrence of each unique element
    my %seen;
    defined($seen{$_[$_]}) or $seen{$_[$_]} = $_ for 0..$#_;
    # For each unique element, stick it on the front of each
    # of the no-repeat-combos of the rest
    map {
      my $first_pos = $_;
      my @rest = @_[ grep {$first_pos != $_} 0..$#_ ];
      map [$_[$first_pos], @$_], no_repeat_combos(@rest);
    } (sort {$a <=> $b} values %seen);
}

sub nrc_iter {
    # Base cases get assigned to an array, which the iterator shifts t
+hrough
    my @base_case = ([@_]);
    return sub{ shift @base_case } unless @_ > 1;
    # Find the first occurrence of each unique element
    my %seen;
    defined($seen{$_[$_]}) or $seen{$_[$_]} = $_ for 0..$#_;
    # For each unique element, stick it on the front of each
    # of the no-repeat-combos of the rest
    my @arg_list = @_;
    my @sub_iter = 
      map {
        my $first_pos = $_;
        my @rest = @_[ grep {$first_pos != $_} 0..$#_ ];
        sub {
          my $recurse = nrc_iter(@rest);
          my $set;
          no warnings 'uninitialized';
          sub { ($set = $recurse->()) ? [$arg_list[$first_pos], @$set]
+ : () }
        }
      } sort {$a <=> $b} values %seen;
    # Below here is boilerplate: if you've done the above steps right,
+ just plug
    # this in, and it works. It returns the first iterator from the li
+st that
    # returns anything.
    # Grab and unwrap an iterator from the list
    my $iter = (shift @sub_iter)->();
    return sub {
        my $rval;
        $iter = (shift @sub_iter)->()
          until ($rval = $iter->() or @sub_iter == 0);
        return $rval;
    }
}


for ([1], [1,1], [1,2], [qw(a b a)], [qw(a b b a)]) {
  print "=== @$_ ===\n";
  my $i = nrc_iter(@$_);
  print "  @$_\n" while $_ = $i->();
}
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