my @hashes = (
    { child_fh => $obj1 },
    { child_fh => $obj2 }
);

my @readable = $s->can_read(1);
for (@readable) {
    my $index = -1;
    while (++$index > -1) {
        last if ($hashes[$index]{child_fh} == $_);
        die("object not found in $hashes!\n")
            if ($index == $#hashes);
    );

    # $index now contains your array index, '3' or '5'
    # from your example.
    print $hashes[$index]{child_fh}, "\n";
}