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

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

sub process_rands {
        my $sentence = shift;
        while (1) {
            last
               unless $sentence =~ s!###RAND\{([^{}]+)\}!get_rand_arr_
+el($1)!e;
               print $sentence."\n";
        }
        return $sentence;
}


sub get_rand_arr_el {
        warn Data::Dumper->Dump([\@_],[qw(*_)]),' ';
        my @array         = split '\|',$_[0];
        my $randomelement = $array[ rand @array ];
        return $randomelement;
}
[download]

@_ = (
       '3a|3b|3c|3d'
     );
  at play.pl line 18.
some text ###RAND{1a|###RAND{2a|2b|2c|3c|2d}|1b|1c|1d} some text
@_ = (
       '2a|2b|2c|3c|2d'
     );
  at play.pl line 18.
some text ###RAND{1a|2b|1b|1c|1d} some text
@_ = (
       '1a|2b|1b|1c|1d'
     );
  at play.pl line 18.
some text 1d some text
some text 1d some text
[download]

while (1) {                                                           
    last                                                              
       unless $sentence =~ s!###RAND\{([^{}]+)\}!get_rand_arr_el($1)!e
+;
       print $sentence."\n";                                          
}
[download]

The  while (1) { last unless ... } loop is more concise and clear and IMHO better if written
    1 while $sentence =~ s!###RAND\{([^{}]+)\}!get_rand_arr_el($1)!eg;
Just replace the  1 with  print $sentence."\n" if the debug print is needed.

Also: Use of the  /g modifier with the  s/// substitution allows killing multiple birds (substitutions) with one stone (loop pass).

Update: Also: Lana: Be sure to test all this thoroughly. Otherwise, you may end up having even more trouble with the planets!

---

Give a man a fish:  <%-{-{-{-<

Oh my! Now I see what you mean :) There's really something wrong with planets today!

I didn't take from the first time because you offered slightly different way, but I was stuck with catching the last match occurence, because I need it to work in one more part of my other script.