You don't need to escape the } within a bracketed character class. You do need not to accept any opening { within your bracketed character class. 
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]
yields 
@_ = (
       '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]

In reply to Re^3: Regex for replacing multiple nested matches by clueless newbie
in thread Regex for replacing multiple nested matches by Lana

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