use strict;
use warnings;

print join "\n", comb( 'A' .. 'C' );

sub comb {
    my @c_out = ();
    push @c_out, permute( '', $_, @_ ) for ( 0 .. $#_ );
    return @c_out;
}

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