This example comes from perldoc -f fcntl:

use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);

$flags = fcntl(REMOTE, F_GETFL, 0)
    or die "Can't get flags for the socket: $!\n";

$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
    or die "Can't set flags for the socket: $!\n";
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REMOTE there is the socket handle. You may need to dereference if you have a lexical handle ( *{$sock) ).