I'm probably missing something fundamentally simple here, but I can't work it out....

I'm trying to cache database results by putting the return of fetchrow_hashref into a hash, single thread works fine but when I share the cache hash I'm getting an "Invalid value for shared scalar" error...

Example of what I'm trying:

my (%aCache)    : shared;
my ($cacheHit)  : shared = 0;
my ($cacheMiss) : shared = 0;

sub Do_Stuff () {
    my ($Id,
        $aDate,
        $aValue) = @_;
    
    my ($dbh, $sth, $row);
    # Check if the required information is in the cache before doing a
+nything else.
    my ($cacheKey) = sprintf("%s:%s:%s", $Id, $aDate, $aValue);
    if (exists $aCache{$cacheKey}) {
        lock ($cacheHit);
        $cacheHit++;
        #
        #  Put cached data in $row
        #
        $row = $aCache{$cacheKey};
    } else
    {
        {
            lock ($cacheMiss);
            $cacheMiss++;
        }

        my $dbh = DBI->connect("Whatever DB",'Default', '')
            or die "Can't connect to Whatever DB database: $DBI::errst
+r\n";
    
        $sth = $dbh->prepare(
            "SELECT a, "
                 . "b, "
                 . "c "
           . "FROM mytable "
           . "WHERE a = ? ")
            or die "Couldn't prepare statement: $dbh->errstr";

    
        $sth->bind_param(1, $Id);
        $sth->execute();
        #
        # Store new row in the cache.
        #  This causes an "Invalid value for shared scalar error:
        $aCache{$cacheKey} = $sth->fetchrow_hashref();
        #
        #  Put the result in $row
        #
        $row = $aCache{$cacheKey};
        $sth->finish();
        $dbh->disconnect();
    }
}
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