my @input = ( 'a' .. 'e' );
my $max_subset = 3;
my $iter = NestedLoops(
    [ sub { [0] }, ( sub { [ 0 .. 1+$_ ] } ) x $#input ],
    sub {
        return -1  if  $max_subset < $_;
        return  1  if  @_ == @input;
    },
);
my @slots;
while(  @slots = $iter->()  ) {
    my @subsets = ('') x @input;
    $subsets[$slots[$_]] .= $input[$_]
        for 0 .. $#input;
    print "@subsets\n";
}

##</code><code>##

abc de
abc d e
abd ce
abe cd
ab cde
ab cd e
abe c d
ab ce d
ab c de
ab c d e
acd be
ace bd
ac bde
ac bd e
ade bc
ad bce
ae bcd
a bcd e
ae bc d
a bce d
a bc de
a bc d e
ade b c
ad be c
ae bd c
a bde c
a bd ce
ae b cd
a be cd
a b cde
a b cd e
ae b c d
a be c d
a b ce d
a b c de
a b c d e

##</code><code>##

sub NestedLoops
{
    my( $loops, $when )= @_;

    my @list;
    my $i= -1;
    my @idx;
    my @vals= @$loops;
    my %freq;

    return  sub { return }
        if  ! @vals;

    return  sub {
        while( 1 ) {
            # Prepare to append one more value:
            if(  $i < $#$loops  ) {
                $idx[++$i]= -1;
                local( $_ )= $list[-1];
                $vals[$i]= $loops->[$i]->( @list );
            }
            ## return   if  $i < 0;
            my $act = -1;
            while(  $act < 0  ) {
                # Increment furthest value, chopping if done there:
                while(  @{$vals[$i]} <= ++$idx[$i]  ) {
                    --$freq{ $list[$i] };
                    pop @list;
                    return   if  --$i < 0;
                }
                $act = do {
                    --$freq{ $list[$i] }
                        if  0 < $idx[$i];
                    local $_ = ++$freq{ $list[$i]= $vals[$i][$idx[$i]] };
                    $when->( @list );
                };
                return @list
                    if  0 < $act;
            }
        }
    };
}