I think you should be using "match global" instead of split. Is this what you want?:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $str = "902 M 903 Textmessage 904 PO 905 S 906 VAS 907 10 908 3629 
+909 85290200429/TYPE=thanos\@test.com 910 NA 911 NA 912 NA 913 NA 914
+ NA 917 0 918 NA 919 Wed,_01_Feb_2017_19:56:23_GMT 922 NA 923 PO 924 
+NA 925 NA 926 07594d85 927 100 928 20170202035623000+08 929 201702020
+35623000+08 930 NA 931 85260531042/TYPE=thanos2\@test.2.com 932 1 934
+ 258;3259 920 NA 921 NA 935 NA 936 NA 938 NA 939 NA 940 thanos-local 
+942 NA 944 NA 945 4880 946 NA 948 NA 950 454000000927816 953 NA 954 1
+3 955 5.3.0 956 NA 957 07594d85 958 NA 961 13 981 NA 982 0 983 852902
+00429/TYPE=thanos3\@test.3.com 984 Wed,_01_Feb_2017_19:56:23_GMT 985 
+RegularThanos 986 TEST 987 NA 988 NA 991 NA 992 NA 993 NA 994 1234567
+89 995 NA 996 NA 997 NA 998 NA 603 0E552E92 602 0 617 NA 618 NA 621 N
+A 635 NA 636 NA 637 NA 638 NA";

# Create pairs every three digits
my (%hash) = $str =~ /\s*(\d{3})\s+(\S+)/g;

foreach my $key ( sort {$a<=>$b}keys %hash )
{
   print "$key => $hash{$key}\n";
}

__END__
602 => 0
603 => 0E552E92
617 => NA
618 => NA
621 => NA
635 => NA
636 => NA
637 => NA
638 => NA
902 => M
903 => Textmessage
904 => PO
905 => S
906 => VAS
907 => 10
908 => 3629
909 => 85290200429/TYPE=thanos@test.com
910 => NA
911 => NA
912 => NA
913 => NA
914 => NA
917 => 0
918 => NA
919 => Wed,_01_Feb_2017_19:56:23_GMT
920 => NA
921 => NA
922 => NA
923 => PO
924 => NA
925 => NA
926 => 07594d85
927 => 100
928 => 20170202035623000+08
929 => 20170202035623000+08
930 => NA
931 => 85260531042/TYPE=thanos2@test.2.com
932 => 1
934 => 258;3259
935 => NA
936 => NA
938 => NA
939 => NA
940 => thanos-local
942 => NA
944 => NA
945 => 4880
946 => NA
948 => NA
950 => 454000000927816
953 => NA
954 => 13
955 => 5.3.0
956 => NA
957 => 07594d85
958 => NA
961 => 13
981 => NA
982 => 0
983 => 85290200429/TYPE=thanos3@test.3.com
984 => Wed,_01_Feb_2017_19:56:23_GMT
985 => RegularThanos
986 => TEST
987 => NA
988 => NA
991 => NA
992 => NA
993 => NA
994 => 123456789
995 => NA
996 => NA
997 => NA
998 => NA
[download]

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