croak "end of file" unless $n;Does that sound reasonable?

Reasonable, if a little unfriendly.

I wonder whether you might change it so that that at end of file, it returns whatever it got:

sub get {
    my($self) = shift;
    my($len) = @_;
    my($x, $n, $o) = ("", 0, 0);
    my($sock) = $$self;
    do {
        $n = $sock->sysread($x, $len - $o, $o);
        croak $! unless defined $n;
        $o += $n;
    } until $o == $len or $n == 0;

    return $x;
}
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But maybe there is no circumstance in which getting less than is expected (how does it know how much to expect?) is a recoverable error?