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## Filtering tye's "combinations" (power set) iterator:
my $iter = combinations(@S);
...
    next unless @c == $K;
    ...
}

## Using tye's Algorithm::Loops:
NestedLoops(
...
    ( sub { [$_+1 .. $#S] } ) x ($K - 1),
    sub { my @c = @S[@_]; ... }
}

sub combinations {
    my ($num, $arr) = @_;
...
        return @$arr[@pick];
    };
}

my $iter = combinations( 3 => ['a' .. 'f'] );

while ( my @c = $iter->() ) {
    print "@c\n";
}