You're suffering from buffering - that program acutally print()s "nothing yet" immediately but you won't see it till the program ends because your output is buffered. try this: 
#!/usr/bin/perl -w
use strict;
use Symbol;

$|=1; # turn off buffering

my $sym = gensym;
open($sym, "perl -e 'sleep(5); print qq!Piped output!;' |") or die $!;

my ($rin, $rout);
$rin = '';
vec($rin, fileno $sym, 1) = 1;
my $changed = select($rout = $rin, undef, undef, 0);
print "select()'ed: changed=$changed\n";
if ($changed && vec($rout, fileno $sym, 1) == 1) {
        my $buf;
        sysread($sym, $buf, 4096);
        print "[$buf]\n";
} else {
        print "nothing yet\n";
}

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In reply to Re^3: How to tell if a pipe opened for reading has data to be read by Joost
in thread How to tell if a pipe opened for reading has data to be read by Anonymous Monk

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