A hash of hashes is exactly appropriate to your problem. The code below counts the number of times each plate has been seen, but if vehicle information needed to be associated with plates then that could become the value rather than a count:

use strict;
use warnings;

my %plates;

while (<DATA>) {
     chomp;
     next if ! /(\w+)\s+(\d+)/;
     
     ++$plates{$1}{$2};
}

for my $group (sort keys %plates) {
     print "$group ", join ', ', sort {$a<=> $b} keys %{$plates{$group
+}};
     print "\n";
}

__DATA__
ABC 334
NTR 557
ABC 442
FTE 442
HHR 443
NTR 554
ABC 123
[download]

Prints:

ABC 123, 334, 442
FTE 442
HHR 443
NTR 554, 557
[download]