I don't even remember what got me thinking that IO::Socket's can be used as file descriptors

Technically they can be used like handles (not descriptors), they're subclasses of IO::Handle. But as you discovered, doing a readline on them is blocking; from my experience I very much prefer non-blocking operations, and those are IMHO best abstracted via an event-based interface.

In terms of example code, I should have been more specific: a Short, Self-Contained, Correct Example.

I seem to have replied but forgot to post it... duh. Anyhow, I'm thinking of sticking with IO::Select and it looks very easy. I create a new Select object initialized to my TCP connection, then I can do a can_read with a timeout, and if it doesn't time out I can use ->getlines to suck up what was sent. If that's all that I was expecting {the returned stuff is terminated by a line with just a ".", as with RCS and unix-mail} I'm done, otherwise I go back and can_read again. Either I get all I needed or a can_read returns a timeout. Seems pretty easy... I can handle the recovery code when it times out. THANKS!!

I'm thinking of sticking with IO::Select and it looks very easy. ... then I can do a can_read with a timeout, and if it doesn't time out I can use ->getlines to suck up what was sent

No, unfortunately it's not that easy. The reason is that can_read will fire even if there's only a single byte waiting to be read, and readline and friends will still hang because it's looking for a full line, which the server might not have sent.

srv.pl:

use warnings;
use strict;
use IO::Select;
use IO::Socket;

my $sock = IO::Socket::INET->new( Listen=>1, ReusePort=>1,
    LocalAddr=>'127.0.0.1', LocalPort=>1235 ) or die "sock: $@";
my $sel = IO::Select->new($sock);

while ( my @ready = $sel->can_read ) {
    for my $fh (@ready) {
        if ($fh == $sock) {
            my $cli = $sock->accept;
            print "New client\n";
            $sel->add($cli);
            syswrite $cli, "x";
        }
    }
}
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cli.pl:

use warnings;
use strict;
use Data::Dumper;
use IO::Select;
use IO::Socket;

my $sock = IO::Socket::INET->new(
    PeerAddr=>'127.0.0.1', PeerPort=>1235 ) or die "sock: $@";
my $sel = IO::Select->new($sock);

while ( my @ready = $sel->can_read(1) ) {
    for my $fh (@ready) {
        print "Attempting to read...\n";
        # all of these hang!
        #print Dumper($fh->getlines);
        #print Dumper($fh->getline);
        print Dumper(scalar <$fh>);
    }
}
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Note these aren't complete examples as they are oversimplified and lack error handling. Anyway, sure, you could implement your own routine to read the socket byte-by-byte until you've got a full line, but that's a wheel that's been re-implemented a million times (I've done it several times myself). Again: I strongly recommend you use a library that already provides this functionality!