Not to recommend this, but I want to bring to your attention that you can do this with split, perhaps you haven't thought about this yet.

use Data::Dumper;

my $str="OP=SiDarkRefRead&DEV=SI_PHOTO_DEV&CMD=READ&STATUS=PASS&READ_D
+ATA={ 3 15 0 255 50000 1000 [0] = 1 [1] = 2 [2] = 3 [3] = 4 [4] = 5 [
+5] = 6 [6] = 7 [7] = 8 [8] = 9 [9] = 10 [10] = 1 [11] = 2 [12] = 3 [1
+3] = 4 [14] = 5 [15] = 6 [16] = 7 [17] = 8 [18] = 9 [19] = 10";

my @p = map { $_ =~ /\[/ ? $_ : () } split /(\[\s*\d+\s*\]\s*=\s*\d+)/
+, $str;
#             ^ only want [dd]=dd          ^ split & capture

print Dumper(\@p);
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And the output is as expected -

$VAR1 = [
          '[0] = 1',
          '[1] = 2',
          '[2] = 3',
          '[3] = 4',
          '[4] = 5',
          '[5] = 6',
          '[6] = 7',
          '[7] = 8',
          '[8] = 9',
          '[9] = 10',
          '[10] = 1',
          '[11] = 2',
          '[12] = 3',
          '[13] = 4',
          '[14] = 5',
          '[15] = 6',
          '[16] = 7',
          '[17] = 8',
          '[18] = 9',
          '[19] = 10'
        ];
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