my $string = 'some text ###RAND{1a|###RAND{2a|2b|2c|###RAND{3a|3b|3c|3d}|2d}|1b|1c|1d} some text';

print process_rands($string)."\n";

sub process_rands {
        my $sentence = shift;
        my $count = () = $sentence =~ /###RAND/g;
        for (1..$count){
               $sentence =~ s!.*###RAND\{([^\}]+)\}!get_rand_arr_el(split(/\|/,$1))!eg;
               print $sentence."\n";
        }
        return $sentence;
}


sub get_rand_arr_el {
        my @array         = @_;
        my $randomelement = $array[ rand @array ];
        return $randomelement;
}

##</code><code>##

some text ###RAND{1a|###RAND{2a|2b|2c|###RAND{3a|3b|3c|3d}|2d}|1b|1c|1d} some text
some text ###RAND{1a|###RAND{2a|2b|2c|3b|2d}|1b|1c|1d} some text
some text ###RAND{1a|2d|1b|1c|1d} some text
some text 1c some text