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y @AssignmentIDs = ();
my $GetAssignmentID_sth=$query_dbh->prepare($SQL);
...
}
my $AssignmentIDs = join(",",@AssignmentIDs);
$GetAssignmentID_sth->finish();

sub read_sql {
    my ($dbh, $sql, $callback) = @_;
...
});

my $AssignmentIDs = join ',', @AssignmentIDs;