This example show how to get the peer username from a unix socket. (getpeereid SO_PEERCRED AF_UNIX)

getpeereid.pl

# See also
# http://www.perlmonks.org/bare/?node_id=881003
# http://www.freebsd.org/cgi/man.cgi?query=getpeereid
use strict;
use warnings;
use IO::Socket::UNIX;
use Socket qw( SO_PEERCRED SOL_SOCKET );

if (@ARGV) {
  IO::Socket::UNIX->new(Peer => $ARGV[0], Type => SOCK_STREAM, Timeout => 10) or die $@;
}
else {
  my $socket = "/tmp/test-peer-id-socket";
  unlink $socket or die "rm $socket" if -e $socket;

  my $server = IO::Socket::UNIX->new(Type => SOCK_STREAM, Local => $socket, Listen => SOMAXCONN);
  die "Can't create UNIX socket: $!" unless $server;
  print "Connect to server with: $^X $0 $socket\n";

  while(1) {
    my $client = $server->accept or die "Cannot accept: $!";
    my ($pid, $uid, $gid) = unpack 'i*', getsockopt $client, SOL_SOCKET, SO_PEERCRED;
    print "Peer socket user: pid=$pid, uid=$uid, gid=$gid\n";
  }
}