That line is really just shorthand for the following:

while(defined($_ = <$sock>)) {
    print $_;
}
[download]

In other words, the code reads from $sock (I assume that $sock is an IO::Socket, so that the angle brackets are overloaded to do what you'd expect them to) and prints each line read to standard output. (What this is useful for here I'm not sure.)

See Statement Modifiers for more on statement modifiers.