MongoDB/Perl: find_one doesn't return data after unrelated code - mongodb

Question

mongodb is v4.0.5
Perl is 5.26.3
MongoDB Perl driver is 2.0.3
This Data::Dumper output shows what's driving me crazy
INFO - $VAR1 = [
'275369249826930689 1',
{
'conf' => {
'param' => 'argument'
},
'id' => '275369249826930689',
'lastmsg' => '604195211232139552',
'_id' => bless( {
'oid' => ']:\',&�h�GeR'
}, 'BSON::OID' )
}
];
352832438449209345 275369249826930689
INFO - $VAR1 = [
'275369249826930689 2'
];
The second INFO - $VAR1 should show the same content as the first one. This is the original code, which I have (see below) broken down to find the culprit.
ddump(["$userid 1",
$c_identities->find_one({
channel => 'chan1',
id => $userid,
})
]);
my #filtered = reverse
grep { $_->{author}->{id} == $userid } #{$answers};
ddump(["$userid 2",
$c_identities->find_one({
channel => 'chan1',
id => $userid,
})
]);
ddump is just a wrapper for Data::Dumper. If I remove the "my #filtered" line, the second find one again returns the expected result (a MongoDB document). $answers is just a listref of hashes - no objects - from some API, completely unrelated to MongoDB.
So I broke the "reverse grep" code down to see where the culprit is. The say are the two numbers you see between the dumpers above. This is what I can do, to get answer from the second find_one:
for my $answer (#{$answers}) {
say $answer->{author}->{id}, ' ', $userid;
push #filtered, $answer;
}
As long as I do just this, the second find_one delivers a result. If, however, I do this:
for my $answer (#{$answers}) {
say $answer->{author}->{id}, ' ', $userid;
if ($answer->{author}->{id} == $userid) {
}
push #filtered, $answer;
}
I get the output from above (where the second dumper yields no return from the find_one. It's insane - the if-clause containing the numeric eq causes the second find_one to fail! This is also the grep body in the intended code.
What's going on here? How can this have possibly any effect on the MongoDB methods?

Answer 1

Using the numeric comparison operator == numifies the value, but it's probably too large to fit into an integer and becomes a float. It can also just become an integer and lose double quotes when serialized to JSON or similar format. Using eq instead of == keeps the value unchanged.