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if( $tmp > 0 ) {
    $sip_response .= "$line\n";
}

my $sip_response = $response; # Make a copy of the response.
if( $sip_response =~ s{^[^\n]+}[200 SIP/2.0 OK] ) {
...
} else {
    die "Didn't understand the response: $response\n";
}