When you have Capture groups with Quantifiers, only the last match is returned. For example,

use strict;
use warnings;

$_ = '1,2,3,4,5';
print /(,?\d)+/;
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will output

,5
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The solution for the task your shown here (IMHO) is to capture the entire series of numbers, then split on commas:

use strict;
use warnings;

my $str = '12/22/2005 20 Notice of Agenda of Matters Scheduled for Hea
+ring (related document(s)2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15,
+ 16, [17], 18, 19) Filed
 by Fubar, Inc.. Hearing scheduled for 4/11/2013 at 11:30 AM';

my ($series) = $str =~ m/\(Related document\(s\)([\]\[\d, ]+)\)/i;
my @matches = split /,\s*/, $series;
s/\[|\]//g for @matches;

print "dollar amp is $&\n";
print "found " . scalar(@matches) . " matches.\n";
foreach (@matches) {
        print "  $_\n";
}
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