IO::Select is definitely the right thing to use here, but I think your code can be simplified a little. Firstly, there's no need to use $s->add($telnet->fileno()), because add can take a filehandle as its argument. So $s->add($telnet); will suffice. You have a bug related to that: the can_read method will return the same objects that you added to the IO::Select object, and so in your code when standard input is ready for reading it will return the glob reference for STDIN, rather than the file number. That will never test equal to zero. (Its numeric value will actually be the memory address of the glob.)

Secondly, if you know all your handles in advance you can just pass them to the IO::Select constructor rather than adding them one by one. And the can_read call can be incorporated directly into the loop condition. So maybe something like:

use strict;
use IO::Select;
my $s = IO::Select->new(\*STDIN, $telnet, $telnet2);

while (my @ready = $s->can_read()) {
    for my $ready (@ready) {
        if ($ready == \*STDIN) {
            handle_stdin();
        }
        else {
            handle_telnet($ready);
        }
    }
}
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