Test based on documentation:

use strict;
use warnings;

use IO::Select qw( );
use IO::Socket qw( );

my $lsn = IO::Socket::INET->new(Listen => 1, LocalPort => 8080);
my $sel = IO::Select->new( $lsn );

my $no_error = my ($r_ready, $w_ready, $e_ready) =
    IO::Select->select($sel, undef, undef, 2.2);

if ($no_error) {
    print("No error\n");
    print("Num readers ready: ", 0+@$r_ready, "\n");
    print("Num writers ready: ", 0+@$w_ready, "\n");
    print("Num errors  ready: ", 0+@$e_ready, "\n");
} else {
    print("Error ", 0+$!, ": $!\n");
}
[download]

Win>perl a.pl
Error 0:

linux$ perl a.pl
Error 29: Illegal seek
[download]

Timeout returns as an error, but with a useless error message. There's no way to differentiate between errors and timeouts, and $! is only valid for errors.

What about select? It could possibly return undef on error and 0 on timeout.

Turns out it returns -1 on error.

use strict;
use warnings;

use IO::Socket qw( );

my $lsn = IO::Socket::INET->new(Listen => 1, LocalPort => 8080);

my $r_in = my $w_in = my $e_in = '';
vec($r_in, fileno($lsn), 1) = 1;

vec($r_in, 100, 1) = 1 if $ARGV[0];

my $nfound = select(
    my $r_ready = $r_in,
    my $w_ready = $w_in,
    my $e_ready = $e_in,
    2.2,
);

$nfound >= 0
    or die("select: (" . (0+$^E) . ") $^E\n");

if ($nfound) {
    print("nfound = $nfound\n");
} else {
    print("Timeout\n");
}
[download]

Win>perl a.pl 0
Timeout

Win>perl a.pl 1
select: (10038) Unknown error

    Using $^E instead of $! gives
    select: (10038) An operation was attempted on
                    something that is not a socket

linux$ perl a.pl 0
Timeout

linux$ perl a.pl 1
select: (9) Bad file descriptor
[download]

So the documentation for select needs to mention it returns -1 on error and that a timeout is not an error.

And the following line needs to be changed in IO::Select's select:

- if(select($rb,$wb,$eb,$t) > 0)
+ if(select($rb,$wb,$eb,$t) >= 0)
[download]

(Well, you could come up with a more efficient implementation.)

With the fix to IO::Select, the usage would be:

use strict;
use warnings;

use IO::Select qw( );
use IO::Socket qw( );

my $lsn = IO::Socket::INET->new(Listen => 1, LocalPort => 8080);
my $sel = IO::Select->new( $lsn );

my ($r_ready, $w_ready, $e_ready) =
    IO::Select->select($sel, undef, undef, 2.2)
    or die("Can't select: (" . (0+$!) . ") $!\n");

if (@$r_ready + @$w_ready + @$e_ready == 0) {
    print("Timeout\n");
} else {
    print("Num readers ready: ", 0+@$r_ready, "\n");
    print("Num writers ready: ", 0+@$w_ready, "\n");
    print("Num errors  ready: ", 0+@$e_ready, "\n");
}
[download]