for( 'hd_defect-onomatopeia', 'hd_defect-host.foo.bar.com' ){
    if( m{hd_defect-( ([^\.]+$) | (?: ([^\.]+).(.+$) ) )}sx ){
      my $fqdn = $1;
      my $host  = $2;
      defined $host or $host = $3;
      $dom = $4;
       defined $dom or $dom = $fqdn;
       print "fqdn( $fqdn) host( $host) dom( $dom )\n";
    }
}
__END__
fqdn( onomatopeia) host( onomatopeia) dom( onomatopeia )
fqdn( host.foo.bar.com) host( host) dom( foo.bar.com )