use strict;
use warnings;

print join "\n", comb_thormod('A'..'R');

sub comb_thormod {
    my $c_out = [];
    permute_thormod( '', $_, $c_out, @_ ) for ( 0 .. $#_ );
    return @$c_out;
}

sub permute_thormod {
    my ( $str, $depth, $c_out, @chars ) = @_;
    if ( !$depth-- ) {
        foreach (@chars) {
            push @$c_out, $str . $_;
        }
    }
    else {
       permute_thormod( $str . $chars[$_], $depth, $c_out,
                        @chars[ ( $_ + 1 ) .. ($#chars) ] )
         for ( 0 .. $#chars );
    }
}