my @strings = (
    'foo1234bar',
    '1234 5678',
    'abcd9012f123ab',
    '123',
    ' 123',
    '123 ',
);


foreach my $string (@strings) {
    my(@nums) = $string =~ m/(?<!\d)(\d{3,4})(?!\d)/g;
    local $" = ',';
    print "<$string>: (@nums)\n";
}