sorry not to be the right guy to review your regex, but I'd suggest a totally different approach.

As you already extracted rules from the fun RFC plethora and, as you said, there are many complications and dark corners I'd prefere a more verbose but easier to implement and to expand dedicated subroutine or, in ideal world, a dedicated module to do this: and yes perl's ecosystem is nearly an ideal world and we have Data::Validate::Domain but as I read in your homenode: I like to take the most out of core Perl before resorting to external modules. let assume you wont your own solution.. but look carefully at Data-Validate-Domain.t and also to Regexp-Common's plethora of RFC utilities for URIs...

To be precise you must be more explicit on what you wont to validate: hostnames to browse or hostnames for a DNS entry? Infact you mention internatiolised hostname but for first ones you have Unicode and for the latter ACE-strings

..but going on your own, and assuming you are not looking for DNS entries, I'd go with something like

sub validate_hostname{
    my $candidate = shift;
    my ($ascii_only, $verbose, $debug) = @_; # leave room for improvme
+nts and flexibility
    my ($return, $descr);
    
        # non ASCII
    if ( $candidate =~ m/[^[:ascii:]]/ ) {   # but see: https://perlmo
+nks.org/?node_id=11164574
        print "Not ACSII\n" if $verbose;
        # ..accepted?
        if ($ascii_only){
            return wantarray ? (undef,"non ASCII string [$candidate] r
+ejected") : undef;
        }
        # go with another specialized sub..
        validate_hostname_Unicode($candidate,$ascii_only, $verbose, $d
+ebug );    
    } 
    # ASCII
    
    # too long..
    if (length $candidate >= 255){
        $descr =  "[$candidate] is too long (".length $candidate." cha
+rs)";
        print $descr if $verbose;
        return wantarray ? (undef,$descr) : undef;
    }
    # Hostnames might be composed by 1 or more labels (separated by do
+ts)
    unless ($candidate =~ /\./){
        $descr =  "[$candidate] contains no dots";
        print $descr if $verbose;
        return wantarray ? (undef,$descr) : undef;
    }
        # .. more checks for this rule

    # Each label may have at most 63 characteres
    ...
    #..have fun :)
}
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