If you have stripped away the TLD, the last non-dot characters are the domain name. Most likely, a regular expression is enough:

my $tld = public_suffix($hostname)
    or die "Invalid TLD found in '$hostname'";
$hostname =~ /([^.]+).\Q$tld\E$/
    or die "Couldn't find domain name in '$hostname'";
my $domainname= $1;
print "Domain name is $domainname.$tld\n";
[download]

Thanks, that does the job.
I have found another way, just now, using Domain::PublicSuffix

Both methods work well, thanks again!

Using <STDIN> works (perl b3.pl < host.txt), but i prefer to run the script without writing the external file into the terminal.

root@sugzum:~/host# cat b2.pl
 #!/usr/bin/perl
 use feature qw(say);
 use Mozilla::PublicSuffix qw(public_suffix);
 open F, &quot;<host.txt&quot; or die $!;
 while (<F>) {
 $hostname = &quot;$_&quot;;
 }
 close F;
 my $tld = public_suffix($hostname)
 or die &quot;Invalid TLD found in '$hostname'&quot;;
 $hostname =~ /([^.]+).\Q$tld\E$/
 or die &quot;Couldn't find domain name in '$hostname'&quot;;
 my $domainname= $1;
 print &quot;Domain name is $domainname.$tld\n&quot;;
 
root@sugzum:~/ssh2smtp# cat host.txt 
200-206-30-98.customer.tdatabrasil.net.br
root@sugzum:~/ssh2smtp# perl b2.pl
Invalid TLD found in '200-206-30-98.customer.tdatabrasil.net.br
' at b2.pl line 9.
[download]

Sorry, I'm a beginner. I can't figure it out to understand how this works.

I'm experiencing another issue now. How can I get this script to read $hostname variable from host.txt? Tried several solutions, but none worked. I could use $ARGV[0]; but it doesn't suit my needs. I need it to read the hostname from an external file. Thank you!