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

    my $sth = $dbh->prepare( $sql );
    $sth->execute();
    return unless $sth->rows;

    while ( my @row = $sth->fetchrow_array ) {
        $callback->( @row );
    }

    $sth->finish;
}

my @AssignmentIDs; # You don't need the empty assignment.
read_sql( $dbh, $sql, sub {
    my ( $AssignmentID, $UserCount) = @_;
    push @AssignmentIDs, $AssignmentID;
});

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