If I understand what you are asking then:

use warnings;
use strict;

my $ports = '0xFFFBFF0000000000000000';
my $basePort = 1;
my @ports;

$ports = substr $ports, 2;

while ($ports) {
    my $octet = hex substr $ports, 0, 2, '';
    my $index = 0;
    
    while ($octet) {
        next unless $octet & 0x80;
        push @ports, $basePort + $index;
    } continue {
        ++$index;
        $octet = ($octet << 1) & 0xff;
    }
    
    $basePort += 8;
}

print join '.', @ports;
[download]

which prints:

1.2.3.4.5.6.7.8.9.10.11.12.13.15.16.17.18.19.20.21.22.23.24
[download]

seems to be a solution, although I don't see how that relates to the initial port string in your sample code.