Use Regex.

$qname="_sip._udp.www.google.com";
$qname =~ m/(_sip\._udp\.)(.*)/;
$qname1 = $2;
[download]

If you want to replace string "_sip._udp." from the variable, you can use the following code.

$qname="_sip._udp.www.google.com";
$qname =~ s/(_sip\._udp\.)(.*)/$2/;
[download]