You missed one combination, "camel book":

#! perl -slw
use strict;
use List::Util qw[ sum ];

sub Cnr{
    my( $n, @r ) = shift;
    return [] unless $n--;
    for my $x ( 0 .. ($#_ - $n) ) {
        push @r, map{
            [ $_[$x], @$_ ]
        } Cnr( $n, @_[ ($x + 1) .. $#_ ] );
    }
    return @r;
}

our $N ||= 0;

print map "@$_\n", Cnr( $N, @ARGV ) and exit if $N;
print map{ "@$_\n" } Cnr( $_, @ARGV ) for 1 .. @ARGV;
__END__
P:\test>cnr camel perl book
camel
perl
book

camel perl
camel book
perl book

camel perl book
[download]