I don't see a check to see how much can be written without blocking. You can have a write in one direction blocking writes in the other direction, which can lead to a deadlock.

As for golfing, this bit:

139: socket($c,AF_INET,SOCK_STREAM,0)&&connect($c,sockaddr_in$ARGV[1],
+inet_aton$ARGV[0])||die$!;fcntl$_,F_SETFL,O_NONBLOCK|fcntl$_,F_GETFL,
+0 for
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(whose only space is not needed) can be reduced to:

100: -MIO::Socket::INET $c=IO::Socket::INET->new(PeerHost=>"$ARGV[0]:$
+ARGV[1]")||die$!;$_->blocking(0)for
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By the way, if you hardcoded the host and port, you could save a bunch more.

Update:

99:  -MIO::Socket::INET $c=IO::Socket::INET->new(PeerHost,"$ARGV[0]:$A
+RGV[1]")||die$!;$_->blocking(0)for
95:  -MIO::Socket::INET $c=new IO::Socket::INET PeerHost,"$ARGV[0]:$AR
+GV[1]"or die$!;blocking$_ 0for
85:  -MIO::Socket::INET $c=new IO::Socket::INET"$ARGV[0]:$ARGV[1]"or d
+ie$!;blocking$_ 0for
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