I believe your problems stem from the while (my $a=substr $z,$g,1) { line. Since the substr can possibly resolve to "0", that is getting evaluated as false; and the while exits.

You probably want something like

  my $a;
  while (defined($a=substr($z,$g,1))) {
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Find a good discussion on the difference between "if val" and "if defined val".

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But you say you are writing a Gnutella client? Well, the packetsize is well defined for each type. 23 bytes for the header, plus (whatever) for the payload. You should be able to use something like

  my $buff;
  sysread($socket,$buff,23);
  my ($did,$payloadid,$ttl,$hops,$length)=unpack("a16CCCL",$buff);
  my $payload;
  sysread($socket,$payload,$length);
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(no error checking in example).