The source for decode_entities_old, which I thought was just a pure-Perl version of decode_entities, looks roughly like this (I've stripped out some context dependence):

sub decode_entities_old
{
    my $array = [ @_ ];
    my $c;
    for (@$array) {
    s/(&\#(\d+);?)/$2 < 256 ? chr($2) : $1/eg;
    s/(&\#[xX]([0-9a-fA-F]+);?)/$c = hex($2); $c < 256 ? chr($c) : $1/
+eg;
    s/(&(\w+);?)/$entity2char{$2} || $1/eg;
    }
    return @$array;
}
[download]

With this code, and the under-populated hash my %entity2char = ( amp => '&', quot => '"' ), we have

say decode_entities_old '"'; => "
say decode_entities_old '"'; => "
[download]

We would get (essentially) the opposite behaviour if we switched the second and third substitutions; that's what I mean by “This is very particular to the ordering”.

Of course, your example shows that decode_entities (which I guess is implemented in XS—I couldn't find it in the source) doesn't exhibit this buggy behaviour, so I guess that there's a reason that the sub I quoted has _old postpended. :-)

UPDATE: I just noticed that I'd mangled my intended bug-inducer, ", above. I wonder if decode_entities (not decode_entities_old) handles it correctly?

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