# writing $line to the appropriate file for $callid. 
# Filename is assumed to be "log.$callid" and is opened if not open
if (not exists $filehandle{$callid}) {
  open(my $fh,'>','log.'.$callid) or die $!;
  $filehandle{$callid}= $fh;
}
my $fh= $filehandle{$callid}; 
print $fh $line;

##</code><code>##

sub process_sip {
    my ($user_data, $header, $packet) = @_;
    my $asccidata = substr($packet,42);
    my $sip_pkt = Net::SIP::Packet->new_from_string($asccidata);
    my $callid=$sip_pkt->get_header('call-id');
    if (exists $main_hash{$callid}) {
 ...