#!/usr/bin/perl
use Fcntl;

fcntl(STDIN, F_GETFL, my $flags) or die "Couldn't get flags: $!\n";
fcntl(STDIN, F_SETFL, $flags|O_NONBLOCK) or die "Couldn't set flags: $!\n";

$_=<STDIN>;

use Data::Dumper;
print Dumper $_;      # --> prints $VAR1 = undef;