foreach my $move ( $LegalMoves->get_legal_moves( $Piece, $Start ) ) {
    # Do stuff here
}

##</code><code>##

foreach my $Piece (@list_of_pieces) {
    foreach my $Start (@list_of_squares) {
        foreach my $End (@list_of_squares) {
            # You supply the is_legal_move() function!!
            next unless is_legal_move( $Piece, $Start, $End );
            $isLegal{ $Piece }{ $Start }{ $End } = !!1;
        }
    }
}