for ( values %main:: ) {
    my $io = *{$_}{IO} or next;
    my $name = *{$_}{NAME};

    if ( defined fileno $io ) {
        printf "fileno of %6s is %d\n", $name, fileno $io;
    }
    else {
        print "($name is a virtual handle)\n";
    }
}

##</code><code>##

fileno of  stdin is 0
fileno of stdout is 1
fileno of  STDIN is 0
fileno of STDOUT is 1
fileno of stderr is 2
fileno of STDERR is 2
(ARGV is a virtual handle)

##</code><code>##

fileno of STDERR is 2
fileno of  STDIN is 0
fileno of STDOUT is 1
fileno of stdout is 1
fileno of  stdin is 0
(ARGV is a virtual handle)
fileno of   DATA is 3
fileno of stderr is 2