The following two snippets are a bit easier to read:

my @readable = $s->can_read(1);
for my $fh (@readable) {
    my ($index) = grep { $hashes[$_]{child_fh} == $fh } 0..$#hashes;

    die("object not found in \$hashes!\n")
       if not defined $index;

    ...
}
[download]

and

use List::Util qw( first );

my @readable = $s->can_read(1);
for my $fh (@readable) {
    my $index = first { $hashes[$_]{child_fh} == $fh } 0..$#hashes;

    die("object not found in \$hashes!\n")
       if not defined $index;

    ...
}
[download]

Or if you don't need the index, just the object in @hashes:

my @readable = $s->can_read(1);
for my $fh (@readable) {
    my ($obj) = grep { $_->{child_fh} == $fh } @hashes
       or die("object not found in \$hashes!\n");

    ...
}
[download]

and

use List::Util qw( first );

my @readable = $s->can_read(1);
for my $fh (@readable) {
    my $obj = first { $_->{child_fh} == $fh } @hashes
       or die("object not found in \$hashes!\n");

    ...
}
[download]