Interesting idea... but I can't even get that to return an empty string:

#!/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;
[download]

Maybe you can construct a better case (?)

I said "maybe", as in "it's something to try". I have no experience with non-blocking handles. Reportedly, IO::Handle::blocking doesn't even work on my OS.

That said, I do have access to a linux machine, and I'm getting the same result (undef) using STDIN->blocking(0).

Anyhow...   Good to have learned IO::Handle provides a blocking method :)